My dealer explained this to me when I bought it. All the electronics are contained in the plastic part of the key. The removable metal key is used to open the drivers door lock when the battery is flat and the fob button therefore doesn't work.
The metal part can also be used to manually release the plastic key from the ignition slot - that's what the little hole is for, and why you can separate the two parts whilst still inserted in the slot. It won't let you start it, or unlock the steering.
The dealer said a friend of his was a keen scuba diver, and would separate the key parts when going diving. He would then hide the plastic part under a seat, lock the car with the metal key, and could then take that part with him underwater rather than having to hide it somewhere nearby!