The main function of a relay switch is to allow applications that normally require heavy currents to be switched on using minimal currents. In an automobile this function helps to reduce the drain on the car's battery.
Most automobiles have relay switches installed on any item that exceeds 10 amps of load or that requires a manual switch to turn on, such as a car's headlights. If a relay switch is not used to operate the headlights, they require much more current to turn on. This causes overheating in the circuit and drain the car's battery. A relay switch also allows applications to be switched off quickly, thus reducing the risk of overheating and fire when something goes wrong.