They do it to make you spend more time browsing. Shoppers typically get the same stuff every time they get groceries. Over time people learn the layout of their local store and develop efficient patterns to move through it and get everything they want. When the store shuffles everything around they force shoppers to wander around the store and to look at all the shelves carefully for the stuff they actually want. Some percentage of them end up finding new things to buy and spend more money.