In my case, it's for an aesthetic purpose. I want to render out a version of my scene that has the blue/pink normal map look. Blender's standard Normal render pass seems to be red and green. I'm sure there is a functional/technical reason for this, but in my case I only want the pass for the look of it.
I would also prefer to do this with compositing nodes/rendering because I don't want to have to change all the materials in my scene individually just to fake the normals/blue pink look. I need both the combined render and the normal.