Effects of microscopic roughness
The two most obvious features that are affected by the microscopic roughness of materials are visual appearance and sliding friction. A perfectly flat surface will reflect light like a mirror. Roughness will lead to more diffuse reflection, although the connection between observed properties of rough surfaces and typical parametrizations used in computer graphics is not clear.
The friction force that opposes sliding is usually assumed to be proportional purely to the force with which surfaces are pressed together. Presumably at least the beginning of the explanation for this slightly bizarre fact is that most of the friction force is associated with microscopic peaks in rough surfaces, and that the number of these peaks that come into close contact increases as surfaces are pushed together.