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."
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.