dear pip, i'm no grounding expert, just a normal guy who loves to explore, learn, appreciate n share technical knowledge to others..
as far as i know our car body is large enough for the -ve charge to be grounded n dissipated. somebody posted not so long ago that the car body/metal will behave as faradays cage so that the -ve charge is discharge to the metal part of the body not to the real ground/earth.
thats why no vehicle has this kind of strap. but for large vehicle like trucks, the trucks has this kind of grounding strap or even steel chain to discharge the
static which occurs due to air movement/friction to the large surfaces.
so if we are at cold n dry climate it may be useful to discharge the static from the car body to reduce sparks.. fellas do correct me if im wrong..
but do install it and lets us know if it give your car differences n can catch miao miao

never had the chance to install this things before..

I don't understand half of it when I read it first time..
..after reading for the 2nd time..I understand now..