With print-on-demand, books that may only sell a few dozen copies a year can stay in print without the publisher having to worry about printing a full run of copies and being stuck with unsold inventory.
An old friend of mine published their poetry and offered physical copies. I had asked about this because I still thought you had to order, for example, a thousand copies and then hope you would sell them all. It is wonderful that this technology has came to be for writers because now you don't have to worry about being left with an excessive amount of copies. This is especially good for people with small followings or who are new to self-publishing. I believe the more accessible things like this become, the more we can hear from voices who can't afford printing thousands of copies.