Why should a large iron chain be dragged at the bottom of a large tanker?
Once this energy reaches a certain amount. Just spark. It is likely to cause a fire. With a car chain, you can release all the static electricity.
Because when the car is running, because of the friction of the tires, a lot of static electricity will be generated. And the tires are non-conductive, which causes the car to build up a lot of electricity.