It's a simple fact players like to be rewarded, for anything. Though I don't really care for achievements are almost insultingly easy to get. The achievements simply tell you and your friends you've done something that might be very hard.

I know people who like to lie about things they never have done to make themselfs seem more interesting and this includes game related stuff. With Achievements and Throphies its at least pretty hard to lie about those now.

It also shows how dedicated you are at playing the game. Plus some games do give other bonuses for achievements gained, like gamerpictures. Or like in Mass Effect, you get certain stat bonuses for getting certain achievements. In other games some difficult unlock might be tied to an achievement as well.

So it's not all just for points, even though most seem that way.