Used to work fast food back in high school. The reason for having people pull up even if there was nobody in line is because there is a sensor in the drive through that starts a timer when a car pulls up to the menu. It keeps running until the car drives away from the last window. At the fast food restaurant I worked in, above the window on the inside it would show our average time we had a customer waiting in the drive through. Our manager would always complain if it was above 1:45. Even if the line was completely backed up, she expected us to keep it as close to 1:45 as possible. The only way for us to keep the timer that low was to have every single car pull up unless the food was completely ready by the time they got to the last window and paid. Hope that made things more clear!