When you acquire the Username it would become facebook.com/YourPageName (or whatever desired was available, usernames on facebook are subject to availability as they have to be unique)
Needless to say, its helpful in SEO & majorly in Branding your services online.
How to get customized facebook URLs for your page?
Sign in your facebook and go to this link: http://www.facebook.com/Username
- Only Pages with 25 or more fans / likes are eligible for customized url's.
- Once set, you can't change the URL or make any modifications to it. Be careful while selecting.