Google reawakened the Voice app earlier this week after years of stagnation. It didn't really add anything to the experience, though. The new app just looks more modern. Google has pledged to remain committed to Voice, and now there are some hints about what's in store. According to 9to5Google, Google is working on VoIP calls in Voice.
Here's the quote Google provided to 9to5.
The updated Google Voice app doesn’t have VoIP features — it has the same features for making calls as the existing Google Voice app. Hangouts will ring by default in addition to any forwarding phones on your account, just as it did before. (Again, not a change with the new app.) We are working on VoIP integration.
Right now, you can make a VoIP call via your Google Voice number in Hangouts, but Google is trying to refocus Hangouts as a business tool. Getting the Google Voice calls out of Hangouts and into the Voice app makes some sense in that context. This would be different than the way calls initiated from Voice work now—they're routed through your regular voice line.
The VoIP bit reads as an off-hand comment by Google, so it's not clear how this will work. It could mean Hangouts completely loses its VoIP capabilities. Presumably Google Voice voicemail would stop showing up in there as well (that would require some changes to Project Fi too). At least that would make Hangouts a little less bloated. At the same time, the Voice app would become useful. Right now, there's little reason to have it installed.