Why did Snape give the Gryffindor's Sword to Bellatrix to keep?

Question

In Deathly Hallows, we find out that, after Ginny Weasley tried to steal the Sword, Severus Snape gave the sword (or rather, a copy, as we find out), to Bellatrix Lestrange to store in Gringott's vault.

Why did he (acting as a Death Eater) do that? It obviously wasn't of any use to him as Dumbledore's agent to give Bellatrix any sword (fake or not); so he must have done it as part of his Death Eater disguise.

At that time, the Death Eaters didn't know that the sword was instrumental to destroying Horcruxes (or that anyone was after them at all). Why the need to hide and protect the sword, especially so well?

Answer 1

To win their trust and/or divert them from the original sword

It can be assumed that at least Voldemort knew that sword could be used to destroy Horcruxes.

We know two things due to which Death Eaters could suspect that the sword was somehow important:

• Dumbledore's Will tried to give the sword to Harry. Although ministry didn't agree with it, they suspected that sword was somehow special (Remember, they tried to find secret purposes of other items given to the trio). As ministry was now under control of Death Eaters, it's safe to assume that they knew about Dumbledore's Will and their reaction would have been same to conclude that the sword was somehow special.

• Members of Dumbledore's Army tried to steal the sword from Snape's office and they got caught and ended up in detention. There were two real Death Eaters posted there to relay the message to other guys that Dumbledore's Army tried to steal an object which Dumbledore wanted Harry to have it.

(Lots of people who knew that Dumbledore wanted to give Harry the sword even imagined that Voldemort could be killed with the sword.)

Now, come to Snape's mind:

• He knew that the sword was definitely important because Dumbledore wanted him to deliver it to Harry.

• Death Eaters could easily suspect that the sword was important because of previously given reasons and they could appropriate it directly from his office. He needed to protect the sword at all cost in case he was right.

• If he had just hidden the sword, Death Eaters could suspect him.

So, why not create a fake replica and give it to Death Eaters: "Guys, I don't know what this sword fuss is all about.. Dumbledore and kids.. but here this is. Let's keep it in our own vaults." or, "Hey Bellatrix, I have a precious gift for you. It's Goblin-made and it belonged to a famous wizard."

Why Bellatrix?
Well, she was the one who suspected Snape most (beginning of 6th book) and she was the closest to Voldemort.

There were also no points against doing that.

Answer 2

He didn't. From what we know, he gave Bellatrix the fake sword, and kept the real one in hiding for when he placed it in the lake in the Forest of Dean.

Deathly Hallows:

‘So how did the sword get in that pool?’

‘Whoever cast the Patronus must have put it there.’

They both looked at the ornate silver sword, its rubied hilt glinting a little in the light from Hermione’s wand.

Half-Blood Prince:

"And you still aren't going to tell me why it's so important to give Potter the sword?" said Snape as he swung a travelling cloak over his robes. ... "Don't worry, Dumbledore," he said coolly. "I have a plan...."

Answer 3

Found a plausible answer:

1. The de-jure impetus for giving the sword to Bellatrix was the attempt to steal it by Ginny, Neville and Luna.

   “Ah, God bless ’em,” said Ted. “What did they think, that they’d be able to use the sword on You-Know-Who? Or on Snape himself?”
   “Well, whatever they thought they were going to do with it, Snape decided the sword wasn’t safe where it was,” said Dirk.
   “Couple of days later, once he’d got the say-so from You-Know- Who, I imagine, he sent it down to London to be kept in Gringotts instead.”

   So, from the Death Eater perspective/excuse, the reason could have simply been as simple as "our/Voldemort's enemies clearly have the design on this sword which is an extremely powerful artifact of one of the Hogwarts Founders, so it'd be best to remove it from where they almost got their filthy hands on it".

   So the as-proposed-from-Deatheater-Snape perspective goal of giving the sword to Bellatrix for safekeeping would have been to deny it to anti-Voldemort forces.

   An additional possible (no direct canon evidence) reason to present to Voldemort would have been to remind him that the Sword would be a very fine object to make a Horcrux out of. Assuming Snape was supposed to have been aware of Horcruxes. I'm really unsure of this, but it does bear mentioning as a possibility.

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2. Of course, as @SS's answer alludes to, de-facto reason for Snape to propose this was mainly so that the Sword would be "out of sight, out of mind" - if the sword's fake replica is in Bellatrix's vault, nobody would start noticing that the sword is "missing" from the Headmaster's office at Hogwarts when it'll be time to give the real sword to Harry. But that clearly wasn't the story he gave Voldemort/Bellatrix