An unedited book is a poorly written book
Too many writers who self publish overlook the importance of editing. Writers should not try to be their own editors. It's not that a writer can't be a good editor—for someone else. It's that writers can't provide the one-person-removed perspective needed to honestly and strongly edit their own work. There is wisdom in the old saying, "A lawyer who defends himself has a fool for a client." Similarly doctors are cautioned not to treat themselves or their families.
Writers should not attempt to edit their own work. It is nearly impossible to put a cold critical eye to what you write with passion and conviction. All who write fall in love one time or another with a too cute turn of phrase, a stretched metaphor, or a sentence as convoluted as an M. C. Escher print.
Editors are the safety net that catches the errors of fact, style, grammar, and voice. Good editors ask tough questions. They are the people who say, "Prove what you wrote here. Convince me." The best editors are expert at catching what slips past fatigued and frustrated writers.
It's not enough that a manuscript be edited. It has to be copy-edited too. Copy editors often are treated as mere proofreaders. They are not. Proofreaders compare a final typeset version of a manuscript to the original writer-produced manuscript. They look for misspellings, misplaced characters, missing words, etc. For a proofreader right is what was in the original.
Copy editors are the grammar and structure police in the world of publishing. A book that has not been copy-edited will contain the kind of errors that diminish readers' confidence in the author's competency and authority.
Between them editors and copy editors give books needed tightness, organization, and polish.