AFAIK, painting is the actual drawing of the pixels; clipping is a process whereby painting in certain areas is restricted (not performed), perhaps because the area is off the screen or obscured by other layer of graphics. By clipping the graphics we are removing redundant paints and therefore improving performance.
Or we might never have existed at all. Freaky. So we should cherish everything. Even this tiny ad:
a bit of art, as a gift, that will fit in a stocking