with anything turbo it is advised to run at idle for a minute or two to allow the turbo to spool down at its own pace without the oil feed just getting shut off then the engine is cut, this will help you save turbo seals and ensure your turbo has a longer and happier life. Alot of people will buy turbo timers, these keep the engine running for a short time when you shut off the ignition to allow spool down. its just to prevent unnecessary wear and tare basically.