It could be argued that, given the abysmal quality of most self-published books, being printed on paper and bound in an actual physical volume could come to be seen as a mark of assurance that the thing has been read, edited, and proofread by someone other than the author's mother.
(Mind you having said that
- A: My mother is a bloody good proofreader
and
- B: The last real paperback self-published book I attempted to read had an 'Editing and Proofreading' credit on the front matter copyright and printing history page, and, despite it being a second edition, had a spelling mistake on page one.)