Used to be because you were able to get a full week of sales figures on shelves + ensuring that you are able to quickly replenish over the weekend if able/needed to for the next tuesday to hit.
Not as important for media anymore, but old habits are hard to break. You have to keep in mind that the entire retail industry has conformed to this standard and deliveries all across the country have been working with this model for decades already.
Pretty much just to see if it sells well enough to send out new shipments.