Ken,the idea is that if the ground lead from the battery should fail.That is the point where the common ground on the engine block returns back to the battery.Without that lead there is no return current to the battery.If the radio is mounted such that the radio chassis is grounded to the vehicle body and hence connected to the engine block and the radio neg power lead is going to the battery direct as it should then the only path for the ground is through that neg lead to the common point on the block via the radio's chassis.The common point on the block means nothing unless it has a return path to the battery.Lift the neg lead off your car battery and replace it with a piece of 22 ga and hit the starter and see what happens.