Another good reason for the car to idle awhile is to get the exhaust system hot enough to boil off any water in it. Water is a byproduct of combustion and will condense into liquid if the exhaust system is not hot enough. That ends up causing rust although I think some of the newer cars have exhaust systems that are more resistant to that.When the engine starts for any reason, it will run at idle until it is warmed up then it will shut off. I read somewhere that this is actually an EPA requirement because of the catalytic converter, the catalytic converter needs to fully warm up to burn off any unburnt fuel. If the engine shuts off before warming up then unburnt fuel can be left in the converter which is apparently not good for its lifespan.