Normally when you use a mipmapped texture on an object the mip level which is currently rendered is determined automatically by the system based on the scaling of the object.

But in special cases you have to be able to specify programmatically which level is to use. The Sample Level modules serve this purpose.

One example if you want a reflective PBR-like shader controlled by a roughness map. A usual technique for this is to pack multiple differently blurred versions of the same environment map into the mip levels of a cube map. Then you can use Sample Cube Level to reach a specific blurred version based on the current value of the roughness map.

Within 1-2 months we'll add some usage examples of shaders of this kind to the official libraries.