A username, also known as a screen name or handle, is a substitute for your real name. It is similar to a penname, but strictly in an electronic medium.
Usernames can tell you a lot about a person, even if you do not necessarily know the person themselves. It is the first thing other users learn about each other, and we do make judgements based on first impressions. For instance, to borrow the examples from the penname article, a handle like Leggy_rox_my_sox23 is not likely to be taken as seriously as something like Camilla Sandman.
On the PPC Board, while there is no way to completely verify someone's identity, impersonating someone or constantly changing your username so as to be unidentifiable is a very bad idea. Also, repeatedly failing to log in with a username when editing the wiki may attract the attention of the Marquis de Sod, and nobody wants that.