my friends and i keep trading games between us so we have a decent selection on the go at any time, my achievements help me keep track of what i've done in each game rather than loading in and out to check where i am. with 4 profiles and about 30 games between us its soooo much easier having an out of game list of 'stuff you've done'.
also achievements give an element of replay value, which is immensely important when you're unemployed and cant afford new games (like i was for two years). unemployment is probably my reason for getting as much out of each game as i could in that time, which is probably why my gamerscore is 35000+. hurray for unemployment! (sarcasm)