Engines that run on idle mode tend to char oil as the machinery burns them while it’s not on its actual operational mode compared to when you’re driving. Residue from burnt fuel builds up on your engine and eventually damages it. Not to mention it is a total waste of gas as you’re engine’s running but you’re car is going nowhere.
Idling is hazardous for the environment, not to mention your health. Gas emissions especially those trapped in garages can harm your lungs as you are susceptible to inhaling carbon monoxide and other fumes from car exhaust. And would you really want to contribute to the growing city smog? I don’t think so.
The same goes for cooling down the engine after driving before turning it off. Drive easy for a few minutes before parking you car and it will achieve the same effect as idling for a few minutes. Cooling off for most turbo cars is required as oil in the engine will char if suddenly turned off. Let it idle for a maximum of 10 seconds before shutting the engine down.