That pile is taking up space and costing warehouse-rental money and potentially adding to the company's tax bill. If your book was a hardcover, you'll probably be offered the chance to buy a lot of copies at a cut rate. Some of the copies you don't buy will go to remainder houses, companies that buy up such books and sell them by mail at a bargain price. Such as . . Imagine a monster machine hidden away in a back room in a bookstore, quietly and efficiently printing and binding copies of books one at a time as they are ordered at a desk at the front of the store .
Nothing is stored in warehouses except for blank paper waiting to be turned into books.
Nothing is returned to the publisher. You don't have to imagine copies of your books screaming as their covers are torn from their living bodies. Nor do you have to contemplate those beautiful pages becoming sheets of toilet paper. In practice, it doesn't work like this quite yet. There is a company, Sprout, that is putting such Print on Demand machines in bookstores, but that's still something rare.