IMO....because the cap and ignition system used shielded cables to the spark plugs, and the shields were terminated at the distributor cap, in order to complete the shield grounding scheme, the cap had a ground strap to be connected to ground. The ground plan was more likely to be for RFI suppression rather than for occupant protection. Shielded spark plugs, wires and ignition components are used extensively in piston engine aircraft to prevent RFI interference with on board navigation and communication equipment. Just my opinion of course.
BOB RENTON