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

Cuz they're fun and sometimes they hint towards your interests или 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 или characters with it......
somehow i found applebear123 isnt used anywhere, so i use this Имя пользователя 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 или anything, I'm just not attached to it.
TBH I'm probably еще attached to my old username, NomyCake, than I am my дана name because I still get called Nomy pretty frequently.

because mauserfan1910 is a hell of a lot еще 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 Имя пользователя that Ты make sound as cool as Ты personally think it is. Besides, if I made my Имя пользователя 'Liam-something или 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 или even aspects of yours. I mean, Ты can do it simply because Ты 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 фото !!!!

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

Like everyone already said, privacy.

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