A published book will be issued an ISBN. Many retailers insist upon an ISBN as a prerequisite to stocking your book for sale. Moreover, after your book is issued an ISBN, you are required by law to deposit 5 copies of that book with the National Libraries. This in turn ensures that your book is accessible to readers in at least these libraries.
Nowadays many mainstream publishers are looking at self published books to publish these books themselves in a revised edition. A published book, that is carefully edited and nicely presented, increases the chances of this happening.