Someone correct me if I am wrong, HOWEVER:
A VDP (or PDP) is mostly used on an INSTRUMENT approach, no?
If you are doing a non-precision approach, you might want to know how close you can be to the runway, and start down (once you see it: i.e. Visual Descent Point) without overshooting it, and having a "stabilized approach".
Simply put, you wouldn't want to start down from your MDA once you see the runway AFTER the VDP (PDP), because then you might be "diving" for the runway (unstabilized). Better off staying at the MDA til the MAP, then try again, or go somewhere else.

Actually just looked at The AIM.
Too long to qutoe some stuff, but look at Chapt. 5-4-5 under VDP, and also pilot considerations for non-precision approaches.