Publishing a book is important, whether self-published (without imprint) or cooperatively published (with imprint). Yeah, you won’t get rich or earn a lot of money from the book sales, but you will open the door of publication opportunities for yourself.
When you have a manuscript, it’s tough to put it out there to be read. You have limited avenues. If you have a book online, the possibility that publishers or agents will read it is there.
You can keep submitting your poems [or other creative work] to journals, but that’s not really a guarantee that a publisher or agent will contact you after reading one or two of your poems. There are thousands of poets that are published by journals. You are just one of them. But not all of them have books that define their poetics and can be used by publishers and agents as their basis for publication.
You can join book poetry contests, too, whose prizes include republication, promotion, and distribution. A book of good poems is enough to characterize you as a poet. One or two good poems you submit to journals or small contests are not. A self-published or cooperatively published book is your path to a profitable book deal.
The case of fictionist Sergio de la Pava is a good example.