The theory behind it is that my first name is common, and having that one common element shared between so many people that make usernames creates a barrier to gathering information easily. If my name was too unique, then it would be easier to find and identify my activity, but as-is, I am not the only person on the net with my username, so anyone performing research has to spend extra time weeding out those people.
Also, I generally don't post anything I wouldn't stand behind anyway.