It has been said "there are no dumb questions, only dumb answers", so here comes one.(posted 9392 days ago)He does this because he is a zone system fanatic. Or, at least, he knows some of the underlying principles.
His subject is contrasty. He knows that a normal exposure and development will yield a negative that is also contrasty, and will be difficult to print. So he plans on developing less. This will reduce the contrast on the negative: subject highlights that would have been a dense black will be, say, a dark grey. But it also affects, to a lesser extent, the other end of the characteristic curve: shadows that would have been light grey will be slightly lighter, or totally clear. By giving the extra exposure, he knows that he can raise the tone of the shadows back where they would have been with normal development.
The opposite also applies: increasing development ("pushing" the film) gives increased contrast as well as requiring less exposure (increasing the EI).
Of course, he has read Adams and BTZS, and has done extensive testing, and knows how to adjust EI (Exposure Index) and development to give a required CI (Contrast Index), while retaining Zone 0 at density 0.1 above film-base-plus-fog.
On the other hand, perhaps he just knows a couple of rules of thumb.