When To Use Automatic P & N?

Stopped in a line of traffic it isn't such a good idea to engage P if you need the engine idling.
A) as you turn the selector from D/S to P it passes through reverse, flashing the reversing lights on and giving the driver behind you heart palpitations!
B) in the unfortunate event of a silly tw*t shunting you then if the gearbox is in P there is an interlock/ gearbox lock (the one that prevents the car from being moved if immobile) which in many cases is a locking bolt or pin that engages - if you are shunted any distance then apart front the back of your car being mangled the gearbox/ transmission can suffer severe damage too.
C) The opposite of A) above when you want to move off - reversing lights flash on and off as you move the selector