First, I want to clear up some confusion: Google Voice doesn't use wifi or data - it still uses minutes! You need a different app if you want to make "free" calls. Something like GrooveIP, for example. Google Voice alone does not provide that functionality on a phone.
Voice Choice is an app that allows you to pre-select which numbers to call using GV and which to call using the normal cell phone dialer. The difference is what number will show in the caller ID to the call recipient, as well as how the call will be billed to each party.
People use Google Voice for different reasons. Some people like that it gives them a single phone number that will follow them to their cell phone, office, or home phone. The visual auto-transcribed voicemail that is accessible via the internet is a big plus as well. Free texting over data rather than SMS is another advantage. It also provides the equivalent of a calling card for inexpensive international calling, but much more conveniently.
People use Voice Choice when they have certain people they want to call using GV and other people they want to call using their normal cell number. One reason is the free mobile-to-mobile provided by a lot of carriers. If you call using Google Voice, that will count as a land-line call to a Google Voice Access Number - even if the real recipient of your call is another cell phone. You'll be charged minutes for that call. I use Voice Choice to keep a list of people on my contact list that are also Verizon subscribers. That way, I can call them using my free mobile-to-mobile minutes, saving both them and me from using plan minutes for the call.