Why YSK: People seem to, on average, think that a car takes a lot of fuel to start up. In reality, it takes on the order of a few millilitres of fuel to start an engine. That means if your car isn’t equipped with an automatic start/stop system to stop your engine instead of idling, it saves fuel to turn off your engine and start it back up when you need it.
Caveat: air conditioning and radio might not work with the engine turned off.
Scenarios where this might be useful include waiting for trains to pass at rail crossings, waiting for food at drive-throughs, dropping off or picking people up on the side of the road when they need to load stuff, etc. May not be a good idea to use this while waiting at a red light because starting the engine does take time which would annoy drivers behind you when the light turns green.
Some cars are equipped with systems that will automatically stop the engine when you are idling for a while (e.g. waiting for a red light). If yours is, then manually turning off your engine will probably result in reduced fuel savings compared to just relying on the car to do it for you.
I can’t speak for other cars, but my Prius uses the electric assist motor as the starter motor as well. Compared to a regular ICE car that’s a massively stronger electric motor than average starting a smaller than average engine.
My favorite thing about it though is I have the longer hatchback model and if you replace one of the back seats you can fully lie down for car camping. What about the heat you may ask? I can just leave the AC on overnight, the car will start up and use the engine like a generator to recharge the battery then turn back off autonomously. I always keep some spare gas in case but I’m always shocked how little it uses.