Usernames probably come from the multiuser nature of early computers, and some of the early limitations of software implementations (eg no spaces so your homedir could match your username on a fs that supports no spaces).
Aliases or nicknames are a common human choice, which allows one to be represented by a word/name of one's own choosing that portraits you in a light you want, without tying to your real identity which might have other implications (sometimes negative, of course).
