Miscellaneous rambling:
FoMoCo paid for a lot of classes to earn/keep our certs up to date. In one electrical class we were looking at PCMs and discussing the way they are built and the electrical protections (diodes/zeners/MOVs/caps)designed into them to combat stupidity. People install batteries incorrectly/backwards, sometimes they jump a car with the ground clamp on the fuel rail. Anyways we got off on a tangent in one of our discussions and grounding, EMF and RF interference etc.
Car bodies/chassis all have different potentials when on the road. Most of your electrical charge is from road contact. Airflow over the car is also a big cause of differing potentials. Some cars have little spring metal "pads" that contact the hood when it is closed, or they have straps. Hatchbacks sometimes have ground straps. Exhaust pipes have ground straps. Bushings are potential insulators making subframes, axles, engine/trans mounts, body mounts all contributors to buildup of electrical charges. Figure in composite, and or plastic components, and you have another problem to contend with. We were told to picture a car body with thousands of electrical arcs occurring every hour it was driven.
Engineers drive prototypes and they measure the potentials all over the vehicle and take action when required. Without proper grounding you can have constant electrical arcing which causes voltage spikes (KV level high voltage short duration spikes specifically) between components. Besides the damage the spike itself can cause, each spike creates RF which have the potential to interfere with electrical signals. Suspension bushings, mounts and other rubber components are all designed to conduct electricity (to a certain extent) and cut down on KV level potentials.) Eventhough rubber components on vehicles have carbon black in them, they can still be pretty good electrical insulators.
Respect for engineers and all the potential problems they have to find solutions for.