The ever-insightful Horace Dediu on Apple’s retail stores:

Which brings up an obvious question: Why did Apple triple employment at each store? One could answer that many of the newer stores are bigger, so-called flagship stores. But the vast majority of stores are in mall locations which are of limited size. Flagship stores alone cannot account for the increase. One could also suggest that perhaps the stores are open longer and there is a need for more shifts. Again, the vast majority of stores are constrained by the opening hours of malls which, as far as I know, have been steady. Anecdotally, it’s also evident that there are more employees in a store at a given time. There are many more of those colored T-shirts at first glance.

One hint is in the design changes in the stores. Apple has removed shelving, registers and almost all non-Apple merchandise. It has replaced the visible stock with tables on which rest products that can be used. If there weren’t any people in the store, the store would look almost completely empty, just an open space.

But that’s the whole point. The stores are designed to be filled with people. The stores have an open layout because it allows more people to be inside the store at the same time. And the more people the more employees.