In game design people have come to realise that putting achievements in game (basically non existent rewards for often boring and repetitive behaviour) ties in very closely with some kind of psychological compulsiveness that makes us need to play games well beyond their entertainment value just for completeness.
I always thought that it was a gamer thing . . . it just makes sense that people will want to see everything in a game, and especially in the case of games like MMOs (World of Warcraft's achievement system is a good example) you've got a heinously compulsive audience to begin with, so they're quite ready to go out there and grind the same action over and over in order to complete their list of achievements.
What I never thought I'd see is a community of audio book listeners being hooked by exactly the same thing:
I only noticed when I received a message on my iPhone's audio book player that I'd received a badge for listening to a certain time length of audio books at a certain time of the day. Of course, I went online to find out how to earn the rest of the badges and found a whole community of people sharing methods to earn the badges . . . but really . . . how does that make any difference to what books you were listening to? Especially when some of the badges are earnt by listening to the same book over and over again . . .
It's a marketer's dream, but a user's nightmare . . .