If you want to use your Google Voice number for every call, keep the app installed for now

The times, they are a-changin'. Google Voice is slowly but surely being absorbed into Hangouts, and you can do basically everything you used to do in the Google Voice app inside Hangouts now instead. I say "basically" because there's one important function the Google Voice app still offers that Hangouts doesn't — the ability to make calls from your Google Voice number in the native phone dialer.

One of the best features of the Google Voice app today is that it lets you make calls through your phone's dialer, so that every time you make a call it will use the Google Voice number, not the number associated with your carrier. And with the way things work right now in the new Hangouts app, you lose this functionality completely if you uninstall the Google Voice app.