Why do you have usernames?

Weeworld.com has usernames so you don't have to give people your real name. For example my username is flysierra, which obviously isn't my real name.

As the example above demonstrates, usernames are used so you don't have to disclose your real name. They also help keep everyone distinct. Think of how many "Bens" or "Johns" or "Sarahs" there would be on any website if there weren't usernames.