For games? Zilch. Although it does eradicate the Myth 2 uninstall scenario from ever happening, lol.
But it does provide a bit of peace of mind when downloading things from the net. And I am not even referring to the big things, which usually have their own large distribution infrastructure, but the small things that someone might want. You don't have to go to some small website and worry about whether this thing you want to download is legit and checking around the web to ensure it is. You can have peace of mind that it won't shit your registry to pieces, it wont leave crap during uninstall, it wont install in weird areas, etc. That certainly seems to have benefit from point of view for both devs and consumers since smaller devs of smaller software might be able to get a wider audience from that. And that is even without them bothering to through the Windows Store. And that is not even touching on the sandboxed nature of the runtime itself in that you can be sure they wont be messing with other processes on your computer in harmful ways. And that last bit is the part that really affect pc games. Although, I foresee Win32 software to be able to penetrate that sandbox.
So it only benifits computer novices who will download stuff at random? But won't they still download stuff at random? The regitry issue is just replaced. UWP takes a lot of control from the user and the stuff you normal programs do in the registry are just substituted by UWP methods of the same things.
You get some more security from having it signed by MS and having a less user access but those are only issues for folks who have intrinsic issues with being an idiot.
Veterans users are just going to be annoyed at the seizing of control and having previous 'expert' options closed off.