That was for early programs that were poorly written, and only worked properly with slower cpu clock frequencies... quite a few old DOS games were dependant on the cpu clock speed. The whole game was accelerated as you tried them on faster PC, and even some games from the 90s did that (for example, Test Drive III, which had digital images and real 3D models, is unplayable on a recent pc without some kind of "cpu throttling" software).
Now if someone could explain me the "boost" button on stereos in the 80s...
