everything everyone else has said is correct about cost saving, but also sometimes they do sell things at a loss. If product needs to move or it will be thrown out to make room for new things theyll put it on sale at a loss so that they can atleast get something back for it. Its better than taking a complete loss when they have to throw it away, and it can act as a loss leader when you buy other things while there that arent as cheap. Loss leaders are also a way they take a loss on items. For example a grocery store may say milk for 25 cents for a week so people come to buy it and while they are there they get all their other groceries at a normal price which evens out the losses taken on the milk.