The Wykydtron said:
Is it the double Phantom Dancers? It is isn't it?
In case there was a grain of seriousness there, it's actually his ult. If he kills an enemy champion while it's active, all of his cool downs refresh, and most of his abilities stack. Add that to a decent build and you have a recipe for rape, although he's weak to stuns, which is why he's not used much at high levels of play.
OT: Because it's fun, and it involves a level of play that just does not exist when playing against a computer. Even at low levels of play, going against humans is less monotonous than going against bots. At high levels of play, your average team based FPS works out in a symphony of strategy and reflexes. I think it's time for some TF2...