I think I remember a few years ago an article about some larger grocery store tracking movements of shopping carts through the store as a way to track and map customer movements. The idea being to determine common routes, sequence, popularity, time spent per area (product), etc. They could then compare sales per product to traffic patterns, and to exit patterns to cashier placements.

This would then be used (as an example) to improve product placements. And not the way you might think. Oh no, don't put most popular things towards the front. Rather, lay out the store so as to lead people past less frequently visited areas en route to more popular ones as a way to boost sales of the former.