What's in an online name?

I’ve been on any number of Q&A websites, and they all have different policies regarding names. On Askville, you can change your name at any time you want. On Fluther, you can change your name once. On Quora you have to use your real name. I don’t even know what the rule is on some of the other places I’ve signed up.

So what are the advantages of various rules? What is the advantage of making people use their real name? Why make people keep the same name? Why allow people to change names at will? I’d like to focus on advantages of each system, since that is sometimes harder to figure out.

If you know of other systems, please let me know what they are, too.