What use are the wizard familiars?

Question

It seems like every student at Hogwarts has a familiar (a companion animal) – Neville's frog, Harry's owl, Ron's rat, Hermione's half-cat-half-Kneazle, etc...

Harry's owl is at least marginally useful for mail delivery (though people without owls can easily use the school owlery services).

But what are the uses of other familiars?

It can't be to practice school magic lessons, as that would require everyone to have the same animal.

And it can't be for companionship – what the heck would Ron need with a rat companion? (it wasn't a magical talking rat, or at least it wasn't supposed to be until it turned out to be an Animagus in disguise).

I'd prefer a canon answer if available.

Answer 1

While they can be useful, the students’ animals are mainly pets.

Presuming this question is referring to only the students’ pets (excluding, for example, Dumbledore’s phoenix Fawkes, Salazar’s basilisk, and the Dark Lord’s snake Nagini), they’re mostly pets. Owls of course are also a way for students to send messages, though anyone who doesn’t have their own owl can just use the Owl Post, as stated in the question. Having a pet isn’t necessary for class work either, as it’s listed as optional. The list doesn’t stay students may bring a cauldron, their required textbooks, or any other necessary equipment - therefore bringing an animal is optional. Also, some students either never seem to get one or get one later on, such as how Hermione only gets her cat in her third year, and neither Seamus nor Dean (who share Harry’s dorm so he’d likely see their pets at least occasionally) are ever seen with a pet.

Additionally, Ron having a rat doesn’t prove that the students’ animals aren’t pets. There are many Muggles who have rats, gerbils, hamsters, and other small rodents as pets, though they don’t provide the same level of “companionship” as a cuddly pet like a cat. In fact, fish are quite popular pets among Muggles, despite them being confined to a bowl. Ron clearly did see Scabbers as a companion, since when he starts looking sick, Ron’s very concerned and wants to keep him safe, then later is furious at Hermione for getting a cat.

“And what about Scabbers?’ said Ron, pointing at the lump in his chest pocket. ‘He needs rest and relaxation! How’s he going to get it with that thing around?”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 4 (The Leaky Cauldron)}}

Ron does occasionally practice magic on Scabbers, who he would have treated like a normal rat since he was unaware his pet was an Animagus. He didn’t do it frequently enough to provide a useful amount of practice, so it’s unlikely he had a pet for that reason. In addition, it was never required for a class, it was just Ron practicing independently.

“He cleared his throat. ‘Sunshine, daisies, butter mellow, Turn this stupid, fat rat yellow.’ He waved his wand, but nothing happened. Scabbers stayed grey and fast asleep.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 6 (The Journey from Platform Nine and Three-Quarters)}}

When the students actually are required to practice magic on an animal, the kind they need would be provided in class.

“Professor McGonagall marched into the room without giving the slightest indication that she knew Professor Umbridge was there. ‘That will do,’ she said and silence fell immediately. ‘Mr Finnigan, kindly come here and hand back the homework – Miss Brown, please take this box of mice – don’t be silly, girl, they won’t hurt you – and hand one to each student –”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 15 (The Hogwarts High Inquisitor)}}

The only occasion that a teacher ever makes a student practice magic on their pet is when Snape makes Neville test a potion on his toad Trevor, but that was done as a punishment and to intimidate Neville, not as something required for the class.

Owls are the most useful, as Hagrid tells Harry when he takes him for one.

“I know I don’t have to. Tell yeh what, I’ll get yer animal. Not a toad, toads went outta fashion years ago, yeh’d be laughed at – an’ I don’ like cats, they make me sneeze. I’ll get yer an owl. All the kids want owls, they’re dead useful, carry yer post an’ everythin’.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 5 (Diagon Alley)}}

While Hagrid isn’t always the most knowledgeable sort, this is supported by the facts, and animals are Hagrid’s area of experience.

Crookshanks, Hermione’s cat, proved to be quite intelligent, though most students’ cats would certainly not be as useful as him. It’s highly likely that he’s half-Kneazle, which would make him different from a normal cat. Crookshanks personally proved to be quite useful, helping Sirius, and later getting Harry and Hermione into the Whomping Willow.

“Crookshanks darted forwards. He slithered between the battering branches like a snake and placed his front paws upon a knot on the trunk. Abruptly, as though the tree had been turned to marble, it stopped moving. Not a leaf twitched or shook.”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 17 (Cat, Rat and Dog)}}

Sirius says Crookshanks is the smartest of his kind he’s ever met. This proves even further that an average cat wouldn’t be capable of the things he’s done.

“This cat isn’t mad,’ said Black hoarsely. He reached out a bony hand and stroked Crookshanks’s fluffy head. ‘He’s the most intelligent of his kind I’ve ever met. He recognised Peter for what he was straight away. And when he met me, he knew I was no dog. It was a while before he trusted me. Finally, I managed to communicate to him what I was after, and he’s been helping me …’

‘What do you mean?’ breathed Hermione.

‘He tried to bring Peter to me, but couldn’t … so he stole the passwords into Gryffindor Tower for me … As I understand it, he took them from a boy’s bedside table …”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 17 (Cat, Rat and Dog)}}

So, it’s highly unlikely that other cats (especially non-Kneazle ones) would be anywhere near as useful. Most likely, the other students’ cats are like the cats Muggles would keep as pets, and would be kept for the same reason.

JKR says in a Pottermore writing that these creatures are indeed mainly pets.

In a writing on Pottermore, JKR states that the students’ animals aren’t familiars, they’re mainly pets, though owls can also deliver mail. (Though there is a JKR statement on this, I also wanted to show it in textual evidence from the books rather than just copying it.)

Familiars, in the strictest sense, do not exist within the world of Harry Potter. Although Hogwarts students are permitted to bring animals to school with them, the cats and rats we see there are, broadly speaking, pets. Ironically, the animal that acts most like a traditional familiar in the entire series is Mrs Norris, who belongs to the castle’s only non-magical inhabitant, Argus Filch. It is true that owls are sent as messengers within the series, but this is in the context of a highly organised postal service, not unlike Muggle pigeon post.
_{^{Familiars (Pottermore)}}

The students, therefore, would be bringing their animals because they wanted a pet, though the students with their own owls wouldn’t need to use the Owl Post as much.

Answer 2

I can think of a few examples.

In Prisoner of Azkaban, Snape used Neville's toad, Trevor, for a potions experiment (Shrinking Solution). Well, and to terrify Neville in the process.

Neville was pink and trembling. He looked as though he was on the verge of tears.

"Please, sir," said Hermione, "please, I could help Neville put it right --"

"I don't remember asking you to show off, Miss Granger," said Snape coldly, and Hermione went as pink as Neville. "Longbottom, at the end of this lesson we will feed a few drops of this potion to your toad and see what happens. Perhaps that will encourage you to do it properly."

Snape moved away, leaving Neville breathless with fear.

"Help me!" he moaned to Hermione.

[...]

The end of the lesson in sight, Snape strode over to Neville, who was cowering by his cauldron.

"Everyone gather 'round," said Snape, his black eyes glittering, "and watch what happens to Longbottom's toad. If he has managed to produce a Shrinking Solution, it will shrink to a tadpole. If, as I don't doubt, he has done it wrong, his toad is likely to be poisoned."

The Gryffindors watched fearfully. The Slytherins looked excited. Snape picked up Trevor the toad in his left hand and dipped a small spoon into Neville's potion, which was now green. He trickled a few drops down Trevor's throat.

There was a moment of hushed silence, in which Trevor gulped; then there was a small pop, and Trevor the tadpole was wriggling in Snape's palm.

The Gryffindors burst into applause. Snape, looking sour, pulled a small bottle from the pocket of his robe, poured a few drops on top of Trevor, and he reappeared suddenly, fully grown.

Prisoner of Azkaban - page 126 and page 128 - US Hardcover

Ginny Weasley had a Pygmy Puff familiar, Arnold, who rode around on her shoulder and seemed to just provide companionship. (Crookshanks seemed to take a special interest in Arnold) Half-Blood Prince - page 127 and page 221 - British Hardcover

Arabella Figg does a "roaring trade" in half cat/half Kneazle kittens (I'd link to JKR's website, but it's down until later this spring) Anyhow, Kneazles are known for being able to detect unsavory or suspicious characters. We know that Crookshanks did not like Scabbers (Peter Pettigrew as an Animagus) at all. Kneazles can also guide their owner safely home if the owner becomes lost. Fantastic Beasts and Where to Find Them - page 24 - Scholastic

Aside from having a conversation about the Chamber of Secrets to Harry and Ron in CoS, Aragog, who could certainly be considered Hagrid's familiar, after he dies in Half-Blood Prince, provides Professor Slughorn with Acromantula venom, which Slughorn plans to use for profit:

Slughorn seemed to be talking more to himself than Harry now.

‘... seems an awful waste not to collect it ... might get a hundred Galleons a pint ... to be frank, my salary is not large ...’

Half-Blood Prince - page 450 - British Hardcover

and:

‘Of course, it’s difficult for you, who knew him best,’ said Slughorn, who, like Harry, could reach no higher than Hagrid’s elbow, but patted it all the same. ‘Why don’t I say a few words?’

Half-Blood Prince - page 453 - British Hardcover

and:

‘I had him from an egg, yeh know,’ said Hagrid morosely. ‘Tiny little thing he was when he hatched. ’Bout the size of a Pekinese.’

‘Sweet,’ said Slughorn.

Half-Blood Prince - page 454 - British Hardcover

Note that while Aragog denied killing Myrtle or being the monster in the Chamber himself, he would not tell Harry and Ron that the creature in the Chamber of Secrets was a basilisk. Harry and Ron found this out later when they discovered the torn out page from the library book on basilisks hidden in Hermione's hand when she is petrified. So the Aragog scenes provided some action/adventure and humour, but they didn't really serve as major plot points.

I don't consider the movies to be canon, but Hagrid did say about Aragog in Half-Blood Prince, "My best (oldest?) friend, he was."

Ron's tiny owl, Pigwidgeon, didn't serve much of a purpose aside from the usual owl function of delivering mail back and forth, and attracting the attention of girls who thought he was cute ("Oh, look at the weeny owl!")

I can see Nagini as the ultimate familiar: Voldemort turned her into a Horcrux. I don't know if the mere fact Nagini is a Horcrux advances the plot; I can see her as being 1) a Horcrux, and 2) a weapon (the attack on Arthur Weasley in OoTP and on Harry in DH) Now if Arthur had died from Nagini's attack, that would have been a major plot point. That said, I can totally see that others would find Nagini to be a major character who advances the plot considerably, so if you feel this way, just disregard this bit.

And, finally, saving the best for last (again some may not consider the movies canon) but in the epilogue scene in Deathly Hallows 2, Albus Severus Potter's familiar is a white ferret, which has to be an in-joke/jab at Draco Malfoy the Amazing, Bouncing Ferret in Goblet of Fire.

Answer 3

Well since you say these are companions, I believe the primary motivation for having them is just like pets. I mean it's a parallel between the normal world and the Wizarding world. Why do people keep pets like dogs? Just because they like pets, isnt it?

When Hermione goes to buy a pet she says, "I need a pet. Harry has Hedwig and Ron has Scabbers."

This simply signifies that they are just pets and in some cases they are useful, like an owl.