This feature allows you to keep your actual phone number for life, as long as you have a high speed Internet connection and as long as you remain with your service provider. If you change service provider, then you have to resort to phone number portability.
Say you have to move from Los Angeles to the Everglades, you can still keep your phone number thanks to this feature. It saves you the hassle of having to let your new number known to people, and of course to memorize it. Old friends calling you on your old number won't miss you.