Until recently, printing a single copy of book was unlikely. Publishers, in the past, have to publish their books in hundreds or even thousands of copies to avail of the best price possible. After printing the books are then sent to bookstores. Those books that were not sold are stocked in warehouses perhaps waiting to be shipped when orders come in. But if no orders come, this can be a big loss to the publisher as these unsold books are part of their investment. This is probably why many publishers are careful in admitting new authors and experimenting with their formats. They would always want to be sure to get the value of their investment and not waste it.
Print on demand, on the other hand, uses a different process. As the name implies publishers have to print only a single copy of book when the order arises rather than produce it in mass copies. Authors have to understand though that when they choose POD publishing their books would never be available in bookstores. Additionally, authors have to do the marketing themselves and can expect that some book critics would never consider their books as true publishing credits.
However, regardless of the downsides...