What is the practical application for normal mode in Uber material


Working with Uber material I stumbled on a new “normal mode” option in Diffuse and Reflectance channels. According to Maxon documentation, a renderer uses normals and phong tag of the mesh by default, but in Uber material normals can be modified via normal maps, bump maps or custom maps. When switching normal mode to “normal map” in reflection channel all the bumps generated by the normal map disappear, but when it is set to “global” all bumps are seen again.

I think I get it: the normals are now modified and the bumps are basically flattened when normal mode is set to “normal map”. I might be completely wrong though. But what is a practical application for this? Should there be two separate normal maps loaded?