Because all the cool people do it.
Then, the un-cool people do it, become cool, and pass it on to their goldfish.
Then, new people join, see the new cool people and their goldfish, and re-start the cycle.
edit Oh god, my sister finished typing up her registration thing for the forums.
I gotta get to a coffee shop within 10 minutes, or she'll get her account accepted e-mail while I'm in the room.