Originally Posted by Overboost

Clavurion is correct, you can get by without the button but it also provides a LED showing a blinking fault besides being able to turn it on or off. It is already coded for automatic reverse gear activation by default.

For instance, my system (front and rear) is set to activate by reverse but then deactivate by speed/distance. So if I back into a parking spot while waiting for someone and just turn off the engine from ignition position 2 to ignition position 1 to keep the radio alive, the PDC is still active and beeping from the rear. I just press the button to turn it off. Also nice to have an on/off switch to check sensors without having to put the car in reverse to check the system or sensors. I would get one, I think I got mine on fleaBay for $25 and that's the way our cars came from the factory with PDC. Here's one near you for £16.01

https://www.ebay.co.uk/itm/BMW-X5-SE...gAAOSwrxpeOp~O