[Joe Wicked]

If you have a different static charge on your filler cap than the pump you risk a discharge. By grounding the cap, you ensure the cap has the same charge as the frame. When you get out of the car, you will generally equalize the car to ground, and thereby discharging any charge. Think of how many times you have gotten out of a car (espcially in the winter with the heater on) and gotten a small discharge/shock. If the gas cap retains this charge, it can discharge when you touch the gas pump nozzle to the filler cap. If the charge is the same, there will not be any discharge. It is a good practice to touch the gas cap area with one hand and the pump discharge plate with the other no matter what car you drive before pumping. This will eliminate a discharge chance during fill up.