You won't need book distribution services or fulfillment warehouses if your book is marketed by a traditional publisher. Nor for a book in electronic form. But the self-publisher may indeed need what the trade calls distributors to booksellers, book distribution services or fulfillment warehouses in these cases:
you don't have the facilities to store books safely
you don't wish to be burdened with fulfilling book orders
you live outside your main market area
If, for example, you live in Turkey but have the book printed in Hong Kong for a US market, it will make more sense to ship the entire print run to the States for a US company to a. distribute to bookstores and chains and b. fulfill individual orders. How?
You have four options:
Many short-run printers, and those serving the small presses, provide a fulfillment service. Here are some better-known companies: you'll find more through Internet searches.
Many of these have sprung up to the needs of ecommerce. You'll find these listed under 'fulfillment company', etc. on the Internet search engines, but below are a few of some hundreds. The fee structure is 1. one-off setup, 2. storage per month (including insurance), 3. handling per item and 4. returns charges. There is often a minimum monthly fee, with package materials and shipping being charged at cost. You may also need to own the ISBN.
Publishing houses don't of course keep their publications of the premises but employ a chain of companies to get the books into bookstores. Many distributors deal with the larger publishers only, and some require the publisher to produce five books or more per year.
Distribution is an important element of publishing costs: remember to factor it in when deciding on the retail price of your publication.
Most POD publishers will handle book distribution for you, at a cost but conveniently through Ingram and/or other national distributors — a point to bear in mind when selecting your Print on Demand publisher.