Well, the facebook username is very useful, you can also get a username and make an identity of your profile. For example your name is namit you can get the username like facebook.com/namit if this username is taken you should try for facebook.com/namit1234. So these can you make an identity of your facebook profile with your name.
These name will very useful like if you want to tell a specific person like your friends or any person about your profile or send a friend request then you can tell your username like facebook.com/username. but if you don’t know how to get a username read my this post “How to Get a Facebook URL Address for My Facebook Profile“.