What exactly does it do? From what I can gather from a quick search, it evens out the tone of the bridge pup to take away some of the brittleness or harshness.. Does it have anything to do with eliminating hum? Or is it strictly tone based?
"The baseplate (sometimes called a bassplate) is a steel plate that is attached to the bottom of a Strat's bridge pickup, then wax potted to eliminate any chance of microphonic feedback. Its purpose is to attenuate the highs of the bridge pickup somewhat, much like the plate on the bottom of vintage Tele bridge pickups. This fattens up the tone a little for those people who find a stock bridge pickup to be a little too bright."