The key FOB is used for the keyless door locks and is also tied in with the starting circuit as part of a theft deterrent system. A dead FOB and the car will not start. The owners manual recommends changing the FOB batteries once a year to prevent such problems.

I have for lack of a better word, a dummy key, that I keep in my wallet in case I lock the FOB in the car. The dummy key will manually open the door. If you try to start the car with it, the engine turns over for less than a second and dies. The starter cycle needs the FOB to start the car.