Abrading dulls the edge so that it has the strength to hold up under the force of the billet. On top of that, because you're hitting a blunted edge, the shock wave travels cleanly on through the stone. If you pay attention to the angle at which you are holding the piece, a long, wide, thinning flake results.
This style of abrading will remove some small chips from the top side and further help build the platform by "rolling it down" even more. Of course, you could pressure flake and accomplish the same thing, but abrading is both faster and easier.