[silicon212]

The grounding is there to ensure the entire body has electrical continuity. Radio transmitters require an antenna which has a counterbalance (ground plane) in order to operate at max efficiency and keep from being damaged. The antenna you see on the trunk is half the equation - the car itself forms the counterbalance and any electrical grounding issues on a car can inhibit the transmitted signal, with some of it being "reflected" back into the transmitter. Any reflected power is power wasted and range reduced. Enough reflected power and the transmitter becomes toast.

Hence, grounding straps.