Aside from in WoW, I never really cared about achievements. Even within WoW, my interest only went so far; I was after titles and mounts, and there were achievements that provided those. In other games, I really didn't care. It was neat when they popped up, but I didn't seek them out. So why is it, that I now have an account at Raptr, and have it set up to tweet my achievements? Why now, do I suddenly care?
Gamerscore, that's why. I know it doesn't mean anything in the long run. But that doesn't stop me from wanting to 100% all the games I own on 360. I want that number to get bigger. I've even purchased some games for the 360 that I already owned on PC, though they were re-bought simply because the control scheme was more fun on console for me.
Some of the games have avatar rewards when you unlock certain achievements. Again, it's another meaningless thing. It's just a small cosmetic reward that I can choose to display or not. But it's a tangible reward, in a completely intangible setting.
None of it really makes sense to me, but hot damn does it have its claws in me. Now if you'll excuse me, I have some Dungeons to Defend...