the plastic has a lower sound transferrence coefficient than the bare metal or even the minute quantity of air that must exist between the plate and the finish in the case of no plastic spacer. Ergo, the plastic spacer allows the plate to vibrate more freely, contributing valuable tones to the instrument. Without it, the metal plate butts up against the body, and since there is a high sound transferrence coefficient, the vibrations are reflected back into the body, causing crossover distortion and unwanted artifacts to be deposited in the finish.
So basically, always eat your oatmeal or porridge when you are a child, otherwise the loch ness monster will continually come to your house, asking for about three fiddy in change, and starvin' marvin will bring his entire extended family to lay waste to your sanity.