I've been a user since grand central and held on to my same number since. At a time when carriers were still competitive, it was great for switching to whatever company were offering the best incentives and not having to worry about telling everyone your new number. Porting numbers sort of resolved this, but I still don't have to worry about it since I only use my GV number.

GV provided online text messages and voice mail before everyone else. GV also allows you to add multiple lines to the same number. At one point I lost my phone and was still able to receive and make calls from the web and my SO's phone.

Its used for a lot more