I'm a completionist. I like having it on record that I pretty much cleared a game 100%.
It's unfortunate, really. I end up doing mindless tasks/grinding just to get points that basically mean nothing, but what can I say, I've always been weak to this. There's a reason I'm doing my best to stay off WoW.
edit: Currently have my 360 on the TV running Gears 2 (stalemate) matches for the 1999 achievement. Yeah...