Diffuse has been called the backbone of reality. I admit this isn't my statement, I have taken it off Visual Magic Magazine in this article:

Diffuse is essentially the amount of light that hits the surface of an object and is diffused, then reflected at the viewer. In terms of computer graphics, it is the property that will dictate to the renderer exactly which amount of rays to reflect. Thus image maps can be crafted to represent the differing diffusion of a given surface.

On average, and object that you would commonly see will only diffuse somewhere between 60% to 80% of the light that hits it.

Diffuse is the property that defines how bright an object is which essentially dictates the brightness and therefore the overall colour of an object. As a result it really defines what we perceive as reality.