I'm going to quote Boogie Nights on this one...
COLONEL: Think about something that makes you happy, something that also gives some pizzaz . . . y'know?
I'm not suggesting you change your name to Dirk Diggler, but this is the internet. Make something up. The site administrator has chosen to require that...this is standard VBulletin software running here and everything is customizable on it. It's required because someone made it required. They could further require it and disable the display of it on the forum and keep it just for their records.
Regardless, they want that field filled in and you've got to put something in there unless the administrator changes something.