I've noticed in countless films and television shows that when characters stop for some reason-- to pick someone up, for instance-- they turn the car off, only to turn it back on again mere seconds later.
One example I recently observed was in the 1995 Michael Mann film Heat. The scene in mind had Vincent (Al Pacino), stop to pick up his girlfriend's daughter Lauren (Natalie Portman) from the side of the road. The driver of the car turns off the vehicle and Vincent and Lauren have a brief conversation. Upon the conclusion of the conversation, Lauren gets in the back of the car and the driver turns the car back on and the three drive away.
Why turn the car off at all?
To be clear: there are a plethora of examples out there, this is just one of them. I hoped to provide a clip for reference but it's challenging to find a clip of an event so irrelevant to the plot of a film.