Basic Auto electricity 101.
Think about this, Engine, Transmission, Exhaust. All mounted in or on Rubber Mounts. Electrically Isolated from the Frame, Body, Chassis etc. This electrical separation Can and occasionally does cause differential in ground potential and electrical problems. The more modern and complicated the cat the more likely problems can exist. Every car ever made in the last 40 years has some sort of connecting bridge between these two othewise isolated ground planes.
That is all.