Knowing very little about all this, and maybe @skyblond has some better insight. But I have a theory. I think its a hold-over mostly from the past when they used brush strokes. A brush stroke will very clearly look different depending on the direction you do it. Also it will pull paint along sometimes if it crosses its own stroke.
So I suspect the direction and order of strokes comes from the fact that it was very critical with a crush, and now with pencils it matters much less if at all but its just ingrained
int he language at this point.