Imagine dragging a boulder across the surface of a tank or car. The brush (Custom geometry piece) would only affect the more exposed area of the model.
Taking it further it would cool if I could simulate the dropping of objects on my model or dropping my model (That I'm painting on) on a rocky ground damaging it only in the very specific areas it touches the ground. II could simulate dropping a gun multiple times add damage and wear.
Ultimately this would be an extension of the particle brush system.