After this date, it is either illegal for the store to sell them (this is the case in Ireland) or the quality will have deteriorated to the point at which nobody will buy them.
If a product is still on shelves after its sell by date, it will have to be thrown away, which is both costly and wasteful to the store (suppliers must be paid even if stock is not sold).
Shoppers, on the most part, will simply walk up to a shelf and take the front most box of the product they are looking for; this is especially true if they are in a hurry.
Rotation also applies to loose products; in this case, there is usually no set sell by date, and produce must merely look fit to eat.
Some customers are fully aware of the practice of rotation, and will reach towards the back of the shelf in order to get newer (and therefore slightly better) produce.