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).
I'm not sure how they started. Was it because of people feeling their 'honest' self info was being used to identify then control them via highjacking their personal accounts in honor of some ex employer?
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).