It just makes practical sense to reduce the length of lines where possible and have the queues following candy, gum, drinks and other small, but profitable, items.
You might as well make sure you utilize the amount of time your customers spend at the queue.
In addition to this, it is also useful to place most popular items such as bread and milk farther into the store, where possible. This means that your customers have to move through the store and pass more items before reaching their goal.
Small, structural changes like this can make a huge difference.