Why do you think people use usernames instead of real names?

Cuz they're fun and sometimes they hint towards your interests or your personality.

For me it's a privacy thing. Though there are people here I've told my irl nick name to and they use that.

Now on a site like tumblr I'd only give even the nick name out via PM.

It's always privacy and hardly anything else.

privacy is one thing..

and my name is way too common, that if i want to use it, i have to use hell lot of numbers or characters with it......
somehow i found applebear123 isnt used anywhere, so i use this username for many sites..its easy to remember!

Because of privacy and security.

I myself prefer nicknames to my real name in general and therefore have no reason to use my real name over a username. Not that I hate my real name or anything, I'm just not attached to it.
TBH I'm probably more attached to my old username, NomyCake, than I am my given name because I still get called Nomy pretty frequently.

because mauserfan1910 is a hell of a lot more badass than Enya.
One is guns, and imagery of the early 20th century, the other is oh yeah, there was a singer lady named Enya, right?

Well it was designed for the purpose of internet anonymity, for people's privacy to be protected, so now it is common for sign-ins to internet sites. However, there is also a certain romantism to having a username that you make sound as cool as you personally think it is. Besides, if I made my username 'Liam-something or other' it would be pretty lame.

Mostly Privacy

So we can be anonymous without actual being anonymous

Well, privacy is always a thing. Besides that, I guess, as a possible way of hinting towards certain interests, preferences or even aspects of yours. I mean, you can do it simply because you can. See it as way to escape dullness. It is not much different than using various pictures to represent yourself instead of your very own photo !!!!

For privacy. Some peoples won't share any of their personal information in Internet.

Like everyone already said, privacy.

My reason is more so that I like to have this online persona as Zeppie. I like creating a new name for myself to use online.