One of the intentions behind the P, N, uv and ObjectID layers was that they allow (Python) developers to write software that applies custom shading.

For example, we've seen some incredible work from Olivier and other when it comes to non-photorealistic shading and this should enable them to do a lot more. Basically, when you're writing a full shader in source code, be it Cg, OpenGL Shading Language or Renderman Shading Language, you mostly use P, N, uv as the input data. With those new custom channels, one could take the output from Poser and implement a completely custom shading engine or just individual effects on top of it, for example screen space AO or sketch rendering. Or one render unlit plain colors and textures to the custom channels, which then an OpenGL application with the aid of the P and N channels could apply realtime per-pixel lighting on.

UV specifically could be used as just one example in a sketch rendering application to determine the stroke direction.