Arbitrarily increase play time.
I'd feel terrible saying I had a 100% complete game and I was missing some achievements. It depends on what it is. I usually have my own personal achievements in games anyway (beat UT with nothing but the impact hammer, stuff like that).
I really hate achievements, and I've said it all before somewhere, but it doesn't matter really.
It's been around in old games. Can you get "S RANK" ? Can you get the "GOLD MEDAL" ? etc, etc.
Just most of those involve, you know, getting points, or kills, or something. Some achievements are just pointless, and it's like "if you buy more of our games, you have a bigger gamerscore. if you spend more time with this game, you have a bigger gamerscore. (KILL X AMOUNT OF ENEMIES, WIN X AMOUNT OF DEATHMATCHES, etc). Skill isn't even what matters with it, so...I don't really get the point. It means nothing.
Some of them do involve skill, some of them don't, and, meh. I guess it all depends on the person. Some people just love having the game remind them that they're great. Usually I just know that. (ie: I don't need a little box to come up when I full-combo a song, the score is there, the 5 stars flashing with the shit around it is there, that's fine right there, and hell, I'll remember, won't I?)
Comes down to what other people said:
Carrot on a Stick.
100% fetish (which I have)
Keeping the game interesting (sometimes)
Completing sets.