It' a nice way to save a ton of time if you have different objects in a scene, some with shading domains, some with layer lists and some that just have a single shader that you want to have all use the same type of shader. Instead of having multiple shaders for each item, you can simply have them "subscribe" to a single shader using the Reference Shader. That way, when you edit, you can only work on one item and it automatically updates all instances throughout your scene. A perfect example of where to use this is for the Poser/DAZ people skin textures.