I'm a firm believer that every writer needs a good editor. Every book needs the careful attention of someone besides the writer. As a writer, I tend to overwrite. I see the story in my head and painstakingly try to transcribe those images into words. In my attempt to get it perfect and to make it as clear to the reader as it is to me, the manuscript suffers from a bad case of repetition and imagery piled on top of imagery. I know this. I can see it. I'm aware that surgical strikes need to be made, however I don't always know where to make the incision. Thankfully, a good editor does.
I think one of the hardest things to get used to as a young writer is the idea of accepting an editor's suggestions. Writers resist. It's a parental instinct to prevent meddling with our children. With my first book, I resisted quite a bit. Less with the next and even less with the next. By now when I get editor's comments, I usually agree with all of them because once I stopped being defensive, I was able to see that nearly every comment I've ever gotten has indeed been accurate and improved the book. This isn't to say I don't occasionally put my foot down. There are some lines in every book that in my mind define the book and it's style that absolutely must stay whether they make sense to anyone else or not.
My high opinion of an editor's input and contribution to a book is one of my strong objections to self-publishing. It's not a snobbish objection that self-published material can't be great writing, because it can. However, in most cases I believe that self-published books probably aren't living up to their full potential because they haven't been fortunate enough to have that expert eye improve their work. This is why I always tell anyone who asks about self-publishing that it should be a last resort. That said, I'm glad it is a resource. Sometimes it's the only way, especially in industry dominated so heavily by trends. However, if you do go that route...do yourself a favor and at least have an educated reader ruthlessly go through your work and make them tell you what isn't making sense and where you've told the same thing over and over.