Why grounding a battery to car chassis?
The chassis, metal body on a monocoque car is used as the negative wire, or ground.
This saves on needing two cables (supply and return) to each device or light etc. The accountants save on the cost by reducing the number of cables and often by minimizing the cable size used to each device.
So each battery negative is connected to the chassis, you could control the negative instead of the positive but it does depend on the total of what you want to achieve and not with that particular relay which has an override function.
Just remember to ground or provide a return for devices or lights on cars that have fibreglass panels :)
Also take care as many cars before about 1965 were positive earth...