The main function is to stop metal from rattling against metal. It protects and seals the aperture where it goes through the body.
It can be used for example on trucks, where you have a truck fuel filler going up the side of a truck. To protect the neck of the fuel filler tube, Spectrum Rubber moulds a rubber product that goes through the body, seals the side of it and the tube comes out the side.
It stops accidents or damage to the vehicle or truck. It comes in any shape or size, so long as the tooling can get it made.