Undercoating provides rust protection for the underside of a vehicle and helps to dampen sound. It is most commonly employed in parts of the country where salt is used to de-ice roads in the winter or where there is close proximity to the ocean.
This protective layer helps prevent the increased oxidization of metals in the presence of saltwater. In vehicles that already have a rust protection coating (common in some models under five-years-old), the undercoating provides an additional layer of sound cushioning that helps to make the car quieter on the road. It may, however, impact the vehicle's fuel efficiency.