Because people might have their name already taken, and can't think of anything else. That's why I use AngloDoom - no-one has the name. However, there's a 'coolguy37' where-ever you go on teh interwebs, and I doubt they are all the same guy. So, someone who is thirty-seven years of age and is so insecure he must be recognised as 'coolguy' by strangers to have the strength to get up in the morning, might stick an 'x' at the front or back of their name to have their desired screen-name.
Then, people will put multiples or make it vaguely symmetrical because we are mostly picky creatures like that. So, our 'coolguy37' has become 'xcoolguy73x'. Notice, he not only looks insecure, but thoroughly gay.
*Walks off podium*