If the fuel is off, this minimises the risk of damage to persons or property should the engine fire when the prop is turned.
Only if your engine is fuel-injected. If it has a carb, there is more than enough fuel in the carb and the lines for the engine to start and run for at least 30 secs or so (depending on the carb). Of course, that assumes that you were silly enough to leave the mags on... and the mixture out of ICO...
