The key's a service key, and also serves as a back up if you lose your original set while out. The idea is you keep the plastic key itself in your wallet (or somewhere about your person) and leave the plastic fob in the glovebox of the car. In the event of your keys being misplaced, you simply unlock the car with the key from your wallet, get in, mate the key to the plastic fob from the glovebox and it should start the car.