It was due to save fuel - an idea to cut one after landing and taxi in on one such, as the DC-9 like we had at LHR.
A couple of our chaps often cut both near the parking stand and coasted in - Very naughty and frowned upon if one was caught or something went wrong.
I gather as long as the APU had been started and all services were on line then they got away with it.
The engines drive generators and HYD pumps - no engines = no leccy and maybe no brakes
I think anyone doing it today (cutting all power and coasting) would get caught and then tea with no biscuits in the CP office