Important Materials Theory - Photographic Rendering with V-Ray for SketchUp - page 250

Whereas the Standard Material's Reflection layer has two glossiness parameters,
one for the reflections themselves and another that controls the highlights, the V-Ray
Material by contrast (and of course, by extension the V-RayBRDF layer) gives us just
a single reflection glossiness control with which to work.
Note
If we need to, we can unlock the glossiness highlight control on the V-Ray Mater-
ial, in which case things will work exactly in the same manner as the Reflection
layer.
The purpose of these glossiness controls is to help us simulate the various levels
of roughness that can occur at the microscopic level on an object's surface and so
cause the reflections created there to become broken or blurred as the bouncing light
is scattered in numerous directions. Now, although this effect could theoretically be
achieved by means of geometry displacement, the resulting load on both CPU and
memory resources would be intense, and in a 32-bit application such as SketchUp,
running into out of memory crashes would also become a very real possibility.