Figure 1: Subway workers on a "sandwich assembly line". Sandwichs move from left to right.
At Subway's 13,000+ franchises the cashier is the final worker on the sandwich assembly line, to give customers a chance to be tempted to make additional purchases. Consequently, after a customer has paid, the newly-available worker is at the end of the line, farthest from new customers. By what process can you get him to the beginning of the line to help the next customer?
Subway tried having the last worker circle back around to the start, but this required space to pass; and, more importantly, allowed the slowest worker to block others and so eventually to set the pace. Subsequently they considered balancing the line using traditional assembly line-balancing techniques; but when they did time-motion studies, the measurements of work-content varied too much to be useful.
Bucket brigades provide a nice solution here because they balance the line based on the actual task times, not on the average task times (which are made meaningless in this context because of the enormous variance).