Really, its primary purpose is for routing or forwarding incoming calls (and texts) to whatever phones you might have. You obtain a GV number, pass that out as your main contact point, people use that, and you can switch phones at will without having to give out a new number or do porting procedures (by selecting the intended phone to forward to in GV's Settings). Especially with a no-data setup, that's what it was made for. You then also have to develop the habit of calling out in such a way that your GV number is the one you appear to be calling from (call your GV no. first, then the recipient). It's a slight hassle, but you get used to it. No smartphone app involved. There's no way it will save you call plan minutes, unless you're using it with a home or work landline (free domestic long distance) -- which is what I do. Or with a smartphone (Android or iOS, IIRC), there are the apps that work with it, but those still don't avoid calling not using plan minutes. The big thing about that was, back when it first appeared, there were still cell phone plans (such as T-Mobile's My Faves, and I believe AT&T, the old Alltel, and VZW had "calling circle" type plans) where you could put your GV number in as one of your circle (Faves, whatever), and essentially have free incoming and outgoing calls. That was a pretty neat deal, but you notice those types of plans aren't offered anymore, and only those holding on to grandfathered plans have 'em.
So it's not for everybody, but it's handy for those who can use its specific benefits. I'm pretty much all in on it.