reasons of putting numbers with our name or username

I don't personally put in numbers in my usernames these days, but back then, one of the reasons I used to do so was because someone else had gotten the name without any number [example: JasonBourne is taken, so I'll use something like JasonBourne10]. Another reason that I remember putting in numbers with my name would be to put in my birthday as well [ex: Fall1911, 1911 being my birthdate].