I will try and explain. An electrical relay is used when you want to turn on a high current devide like headlights. It only takes a small voltage and very little current to engage a relay. If you used a switch to turn it on it would have to be pretty big to handle the instant turn on current and constant current load. In a relay it has two good sized metal plates that come together when energized and they handle the current load. I hope this helps. Or is it still clear as mud?