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].

First of all, the main reason would be that when they one is new to Internet and try to register to his/her first website, he/she would have probably put his name or nick name and would have already be in use. So they try the same nick name with adding 1 or 2 number in back or front of their nick. Once some peoples start using that, it becomes an habit for them and use that username (with number) everywhere they go. Some peoples don't even realize that their real nick name would have probably not chosen in site. They just put the nickname they created in their first website everywhere, since they got used to it very much. I think probably this is why usernames comes with a number in it.