These are typically magnetic signs that you attach to the outside of a car. They're often used for business advertising, where a business can place a logo, phone number, or other information on a vehicle without permanently altering it. These are convenient because they can be easily removed or swapped out.This could refer to components of a car that use magnets, like the magneto in older ignition systems. In these systems, a magnet is used to generate electricity to create a spark for the engine. Although modern cars generally use electronic ignition systems, the term "magneto" is still sometimes used in the context of older engines.
Some car washes use brushes with magnetic components or features to help clean or maintain the vehicle. These are less common but are sometimes seen in automated car washes.