If you are an author or a small publisher, print-on-demand helps you avoid investments in large inventory i.e. it avoids the need to print a large number of books. Similarly, print-on-demand helps small businesses by reducing costs incurred in printing manuals and reports. Print-On-Demand also allows publishers to make back lists available without large print runs.
For details on how print-on-demand, in particular, and self-publishing, in general, might be of use to you, see which publishing solution may be applicable to you.