Ron being given a love potion is the catalyst to realizing the plot to poison Dumbledore.
Harry having to take Ron to Slughorn because of the love potion was the reason Slughorn shared the wine that ended up being poisoned as an attempt to kill Dumbledore. The encounter with Romilda set that up so Slughorn would have a reason to share his wine with them.
We also learn Merope Riddle used a love potion, and this gives us a firsthand look at one.
Before Romilda tried to use the love potion on Harry, love potions are mentioned a few times before. Weasleys' Wizarding Wheezes sells them, Slughorn shows them to his class, but the most significant is that the Dark Lord's mother used love potions on his father to get him to show interest in her.
“Can you not think of any measure Merope could have taken to make Tom
Riddle forget his Muggle companion, and fall in love with her
‘The Imperius Curse?’ Harry suggested. ‘Or a love potion?’
‘Very good. Personally, I am inclined to think that she used a love
potion.” - Harry Potter and the Half-Blood Prince, Chapter 7 (The House of Gaunt)
The only reason she was able to have Tom was because she used the love potion on his father, but as soon as she stopped, his father left because he didn't actually want to be with her.
“You see, within a few months of their runaway marriage, Tom Riddle
reappeared at the manor house in Little Hangleton without his wife.
The rumour flew around the neighbourhood that he was talking of being
“hoodwinked” and “taken in”. What he meant, I am sure, is that he had
been under an enchantment that had now lifted, though I daresay he did
not dare use those precise words for fear of being thought insane.” -
Harry Potter and the Half-Blood Prince, Chapter 7 (The House of Gaunt)
However, we just hear about this through Dumbledore, and until Ron ends up getting the love potion Romilda meant for Harry, we don't truly get to see the devastating effects a love potion can have on someone. By having Ron under the effects of a love potion, we can see for ourselves how differently Ron acts while under it, and understand the Riddle situation better.
As for why it wasn't an existing minor character, there weren't any who'd plausibly work.
For something like giving Harry a love potion, it would be best to use someone who's basically a throwaway character, and certainly not someone we're supposed to like or trust. It wouldn't have really worked to use one of the girls on the Gryffindor Quidditch team, since after having someone use a love potion on him or Ron, neither him nor Ron would trust them much again. Lavender Brown was already dating Ron, and after the Yule Ball neither Patil twin would have really wanted to date Harry after that. It certainly couldn't have been a Slytherin girl like Pansy Parkinson or Millicent Bulstrode, since most of them actively hate Harry.
Romilda could have new interest in Harry because she's young and just getting into boys.
In addition, most of the other female minor characters are old enough where they've been around Harry long enough that they wouldn't hero worship him or suddenly develop a crush on him. It would be much odder for an older girl to develop a sudden intense interest in him than a younger girl who maybe only just developed an interest in dating and boys.
Romilda doesn't come entirely out of nowhere, her crush is shown before the love potion.
Romilda being interested in Harry was set up from the beginning of the books. She's first seen as the only fourth year in a group of girls who all like Harry who's brave enough to ask him to sit with them.
“You ask him!’
‘I’ll do it!’ And one of them, a bold-looking girl with large dark
eyes, a prominent chin and long black hair, pushed her way through the
door. ‘Hi, Harry, I’m Romilda, Romilda Vane,’ “she said loudly and
confidently. - Harry Potter and the Half-Blood Prince, Chapter 7 (The
Then she attended Quidditch practice, presumably just to get a look at Harry.
“The second group comprised ten of the silliest girls Harry had ever
encountered, who, when he blew his whistle, merely fell about giggling
and clutching each other. Romilda Vane was amongst them.” - Harry Potter and the Half-Blood Prince, Chapter 11 (Hermione's Helping Hand)
She also tried getting Harry to go to Slughorn's Christmas ball without using any love potion first, but resorted to love potions after Harry showed no interest it her, not taking no for an answer.
“At last, he extricated himself from Romilda Vane, who was hinting
heavily that she would like to go to Slughorn’s Christmas party with
him.” - Harry Potter and the Half-Blood Prince, Chapter 14 (Felix Felicis)