Why did quirrel send a troll into the dungeon in harry potter and sorcerers stone?

"On Halloween, Quirrell allowed a troll to enter Hogwarts as a diversion, and sneaked to the restricted Third Floor Corridor where the Stone was being hidden. Severus Snape, who suspected Quirrell, followed him, and was bitten by the three-headed dog Fluffy in the process of trying to head Quirrell off."