Electrical relays are essentially electrically operated switches. They have been used in a wide array of electrical systems for over 200-years. Early classic cars had little need for electrical relays and often even lacked fuses or circuit breakers. As cars became more sophisticated and the electric systems more robust, relays proved an effective means to deliver large amounts of electric current without having to go through a conventional switch.
There are several advantages to using electrical relays when building or adding on to your electrical system and they include: Shorter heavy gauge wire lengths, deliver higher amperage to components, take load off of switches, reduce electrical fire risk. Relays have proven themselves reliable in many automotive applications, including horn, starter and cooling fans. Today most modern cars include dozens of relays for nearly every system on a car.