The number is there because Verizon (or AT&T) needs it to track your account. That's just they way they work. And, no, there isn't anything you can use it for. At least not as far as I can know.
When you go to the My Verizon site, you might be able to use it to log on in lieu of a user name, if you don't already have an associated 'real' phone number. Not sure.