As the dog gains understanding of this command he will soon learn that the faster he sits and waits the sooner he will get to eat. Using the wait command at this time consistently, keeps the dog from diving at his food and communicates to the dog that you are the leader and will only allow him to eat when you say so. As the pack leader that is your position. A dog that is allowed to proceed before the owner may think that he is the pack leader. This small distinction is an important one.
Dogs are much more comfortable knowing who they are to follow.
You have to spend a little time in the car teaching this but the same priciples apply. This command may save the dog's life by preventing him from leaping out of the car and into traffic as soon as the door is opened.