Do it for the statistical reasons (I am really curious): join fandom_counts.

You are not required to do anything, just join. There's no posting and it isn't used for petition reasons or other lofty causes, they're just trying to figure out how many of the 13 million users on LJ are fandom people. They have breached the 10,000 mark, and I’m really curious as to how many fandom-people are out there on LJ.

ETA: Link edited.