It's a convenient way to add 4 channels of major power-control to your Arduino with a minimum of fuss.
If you want to drive an inductive load, or AC, or DC, or mysterious load, you can't beat a relay. It just works. It isn't fast, but it sure is powerful.
This shield plugs directly onto a standard Arduino footprint device, and you have 4 relays at your command.
Note: Please ensure the back of the board is insulated before attaching to an Arduino main board. The green terminal blocks can short out against a metallic surface like the top of the USB connector!