For example, diffuse indirect radiance tends to have be low frequency in space, so we can construct good approximations of it using only a few samples (see for example, voxel cone tracing). Tracing rays is an easy way to sample the geometry of the scene and construct a diffuse GI approximation.
Doing proper reflections and refractions are also not really possible with rasterization pipelines. Game engines do a lot of hacks to approximate reflections and don't really do proper refractions at all. With a ray tracing primitive, these effects become possible. For a good example of what a modern rasterization engine does see http://www.adriancourreges.com/blog/2016/09/09/doom-2016-gra....
Rasterization is very good at generating the visible geometric information in the scene which serves a purpose, but for general purpose lighting we need to be able to sample what's going on in non-visible parts of the scene.
I think having GPU support for ray tracing is exiting as a complement to existing rasterization techniques: You can generate the geometric information and attributes of your visible objects with rasterization and use ray tracing to sample your scene and get accurate light transport effects like reflections and global illumination.
I see that GI and ambient occlusion are generally low frequency in space, but don't reflections and refractions require higher fidelity?
uh, I'm thinking mirror-like reflections, but there are softer reflections, like off a matt red wall (seems GI-like).
That said, just rendering mirror surfaces or refractions doesn't require more than an extra ray per interaction (one in the reflection direction and one in the refraction direction). You can add a texture and get bumpy reflectons or refractions as well.
The difference between these easy effects and the high spatial frequency caustic effects is indirect versus direct lighting. It's very easy to ray trace a a simple transparent model but a bright spot on the floor (e.g. [0]) below a reflective objects requires a lot of ray samples to capture directly.
[0] http://news.povray.org/povray.binaries.images/attachment/%3C...
I am guessing most of this is thanks to NVIDIA's new denoising system. I haven't used it but it's very quick and automatic. Supposedly it sometimes has artifacts but it's quick enough to overlook that.
My feeling is that, like VR and strong AI, we are still a ways off.
OTOH, even if raytracing just gives very low-quality, but acceptable results, it could be a great way to sell more powerful GPUs. The thing is, for low-quality to be accepted, it must be better at something else. (Like, smartphones were less powerful than PCs, but portable and convenient).
What does raytracing have going for it, even without throwing pixar-scale resources at it?
e.g. without the need to fine-tune layers of hacks, it might be better in dynamic, non-designed environments; or, for graphics to be put together with less (expensive) expertise and time.