Full Assed FTW!
I have a friend who'd say: "That sounds vaguely obscene and if there's anything I can't stand it's vagueness!"
As far as I can tell it emboldens the usernames of people you add to the list, so that they stand out in the "Who's Online" list at the bottom of the main page. Like spotting your friends in a crowd because they're wearing bright clothing. Or funny hats.