This is a pure UX question. I did similar research for bank teller lanes in college. I will answer your question with one word - perception.
When you drive up and see 5 cars ahead of you, you might be like "F that I will try somewhere else." They know once you order you will hardly ever leave. So they just want you ordering and don't want to lose your business. Your question has two lanes in the example... Sonic took this to the max as some of their locations have 40+ ordering points. It is all about perception as you are still in line. They just want you to think that the line is as short as possible until you order.
I am not saying the other answers are wrong. They make valid points but they are all secondary to this point. As you can see below, you can drive up and order right away at Sonic and you will be the first person in line ;).
