An unabraded edge is sharp. It uses up the shock from the billet before it can do any good. Without abrading you end up with a crushed edge and a myriad of step-fractures.

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.

Abrading Function
The most important function of abrading is to strengthen the striking platform. Often, abrading from the underside of the blank (the side that the flake comes off of) across the edge will not only strengthen the platform, but will help shape it too. This is especially true if there is some overhang.

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.