This is something that authors often ask themselves. Sometimes they ask me if I think they need an editor. The answer is usually yes.
Traditional publishers want to receive a completely edited manuscript. Yes, they will have their editing staff spend some time with it before it goes to publication, but they still want the manuscript in good shape when they get it.
The owners of most pay-to-publish companies don’t care much whether the books they publish are edited. Most of them do offer editorial services for a fee, however. And most of the authors I talk to say these editing services aren’t worth the money. Professionals recommend that you hire a good book editor before approaching a pay-to-publish company.
If you plan to establish your own publishing company and produce the book yourself, you definitely need to have a credible editor look over your manuscript.
So who needs an editor? The answer is: every author.