I think it is just a deterrent... Although the immobilizer is still the most effective anti theft device in a car... (although in a manual, I guess they can just push it away as there isn't a shift lock!
The dealer in my country (Greece) has fitted an alarm and the sensor pack as a gift
It has a second small led lamp at the front of the dash board and it is embedded into the key. So when you press the lock you hear the (bibbbb) the turn lights are open once. when you close you hear two (bibbb) and again the turn lights are open twice.