And worse yet, the expert may have learned a little trick unconsciously and be unaware that he is doing something that might benefit the novice. Abrading is like that. This is especially true if there is some overhang. Of course, you could pressure flake and accomplish the same thing, but abrading is both faster and easier. The minute scratches on the platform will break up the surface adhesion and the flake will release easier.
Different situations require different abraders.
Rough-out work normally requires a very coarse abrader but will damage the edge of a delicate, nearly completed point. And too much grinding can cause such a dull rounded edge that it becomes impossible to flake at all. Most knappers use a rough rock or two for their grinding stones, but not all. A few use an old, worn out file. Several use old broken grinding wheels.