Why do you need to give a username the first time you create your profile?

Hi,

I'm an Internet teacher and soon, I'll be teaching my students about MSN
Messenger again.

While working with Messenger, my previous group of students came up with a
question I couldn't answer:

Why do you have to define a unique username the first time you create your
profile? It is sort of annoying that you often have to choose the oddest
combinations of letters and numbers to find a unique username and it is even
more annoying that this is the name that is put at the top of your profile.
Why can't you just use your name or nickname? I'd like to explain my
students why that is!

Thanks,

Lenny