For 3D the biggest improvement is you have more pixels to render objects at long distance, which is huge for games with large camera viewing distance. For example, you can spot enemy much easier in Battlefield.
I see your point. I guess it also requires much larger screen. Because if 1 pixel man will be moving on my 27-inch fullhd screen, even though on 4k display it will be 4 pixels, it still be same physical size and not much easier to notice :)