Realistic renders with raytracing assumes accurate models. In the scene, the grass can be a rectangular block with a grass texture applied to it for instance, NN will transform it into an actual grass patch (see example images) similar to how it would look like in real life (not constrained by your 3D geometry / scene setup). So realistic lighting is just one part of the problem space here. A road with an asphalt texture applied on it will need a lot of manual tweaks / material design to look realistic under raytracing regime, NN can look at it and implicitly say "hey, this is a road, and I know what a road looks like under this lighting, so I'm going to render it like this" and it automatically becomes realistic.