The main purpose of the ReferenceImage setup is that the ReferenceImage shader does not receive light or other shader information from within the level. It keeps the image at its pure source:
Illum Shader on the left, ReferenceImage Shader on the right
This tutorial shows how to add raw reference pictures in a level.
This picture shows the difference between the standard Illum shader (left panel) and the ReferenceImage shader (right panel), unaffected by the noise/grain shading.