For one, as a plane for stuff that doesn't particularly want to be planed. This with a flat blade.
Second, there's toothed blades for it, and my chippie friend used that to roughen wood to improve adhesion. So: take a piece of chipboard, glue 1/4" of solid wood on the edge, level it off, then go over the surface with the toothed plane, i.e. the solid wood and 4"-6" of the chipboard surface. This to make the veneer stick better to the edge, both the chipboard surface and wood edge strip. Not doing it works just fine, particularly if using modern glues, and saves an impressive amount of work.
But doing it for hundreds of boards makes for a very impressive handshake that can crush walnuts -- ha did this as an apprentice. I've got that plane now...
Thomas Prufer