If you have a car that comes with a special "valet key," the immobilizer probably shuts down the fuel pump if the car is started without the code. This means the car is going to run only on whatever fuel is left in the fuel line, which will only get it a couple of blocks. Thus the valet key -- valet parkers only have to drive a car very short distances. If they try to drive off with your car, they won't get very far. Neither will any other potential car thief.