Why do people have my username in their siggies?

Why do people have my username in their signatures?

Some signatures may have something like "Hey Guest how are you?"
Even if it's a random person who doesnt know you, don't be alarmed.
It is practically just a code to use in the forum (NOT in chat, it doesnt work) that will say your username, or anyone who is logged on.

For people who are not logged on, it says. Guest.
For people who are logged on, it says your username to log on. Such as "TestAwesome!"; or whatever your given username is.

Code:

{USERNAME}

People use that code to say hi to everyone in the forum, etc.

I hope I helped anyone concerning with it. Any questions, PM me, I'll add it here.