If you don't have a particular web presence or persona that's important to you, then I guess there really isn't a point.
For those that do - in which group I include myself - there is a vested interest in making sure you get the name you want.

Mark

I meant what's the point in having a Facebook "username" in the first place?.

Makes it easier for people to find you. Now you can give out a URL rather than people having to search.