Having first class ray tracing primitives on the GPU gives us an easy way to implement a wide variety of effects, even with a relatively small number of rays. While we are certainly far from getting real time performance in generating images comparable to those made by offline rendering techniques, being able to trace rays into the scene efficiently does allow for new degrees of realism in real time renderers.

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.