Sneak Attack decks cheat enormously expensive creatures onto the battlefield without paying their normal mana costs. The namesake enchantment supplies haste but sacrifices the creature at the end of the turn, making immediate impact especially important.
Emrakul, the Aeons Torn and Atraxa, Grand Unifier provide appropriately enormous targets, while Show and Tell supplies another way to bypass their mana costs. Legacy's Sneak and Show archetype is the strategy's most famous home.