At the risk of sounding clueless, would someone tell me what my benefit of using GV is if I already have Sprint with the unlimited cell minutes?

I haven't really listed very intently to this whole thing, so I'm a little behind.

I know that before you could make calls using your GV number - can we now set our Sprint number as our GV number? If I set my Sprint number as my GV number, do I incur charges for using it?

Right now I'm only using GV for the voicemail feature, but would be interested in learning more.