Why laboriously fold rather than quickly stir? Because the rough mass of starch, fats, and other foreign matter in the base pops bubbles, and the more you rub the bubbles against such a mass, the more bubbles you lose. Simply stirring continuously grates the two phases together and causes a substantial loss of air. Folding has the advantage of disturbing the foam only along the surface where the base is being deposited, and that surface is only disturbed for a single stroke. The result is minimal grating of bubbles against mix, and maximal bubble survival.