Why not do it all yourself?
Well, most authors are too close to their work to see mistakes. They are so into their plot and characters and arguments, they don't see it from the perspective of the reader. This applies to both fiction and nonfiction. Authors miss obvious grammatical errors, awkward sentence construction, incorrect punctuation, misspellings, and monotonous or systemic patterns of writing that leave the reader bored or unsatisfied.
In technical and academic writing, a boring or awkward writing style is certain to lead to frustration, as the reader is already struggling to assimilate technical material.
In fiction and creative writing, too much narration, unsatisfying dialogue, inconsistent plots, poor character development, lack of scene setup, and awkward sentence structure is sure to turn off your readers. You need a pro to catch these and make suggestions and changes in the manuscript BEFORE you send it to the publisher!
I will sum up my editing philosophy in one sentence: The purpose of an editor is to make the writer look good.
Many publishers receive dozens of manuscripts every day. Their trained eyes can immediately spot a bad manuscript, and recognize a good one. The bad ones go right into the trash can!
Here's a little secret I've learned from writing 8 books: regardless of the type of book you are writing, the most important thing about any manuscript is that your narrative needs to flow smoothly so as to hold the reader's interest.
This is what I call the Law of Good Writing. This applies just as well to technical and academic writing as to novels. For example, I just finished reading a physics book by a Professor Emeritus at MIT [name and publisher withheld!]. The book cover is one of the best I have ever seen; the paper, of the very highest quality. The manuscript, even for an academic work, is engaging. Yet the book has many obvious grammatical and sentence errors that should have been caught by a good editor. In this case, a poor editing job makes the professor look bad, and detracts from an otherwise excellent work.
As an author, you need to engage your reader and make it easy for him or her to understand what you are saying. And do you know what? A good editor can do that for almost any book, no matter what the genre.
I always have others read my books; either a paid editor, or, for novels, general readers who would be my target audience.
If you are the type of author who is willing to spend months editing and rereading a manuscript (as I am) it is possible to do it yourself, but the time you must spend is prohibitive. For example, I wrote "The Vibrational Universe" in 3 months. But it took me 9 months to edit it! I went over and over the manuscript more than 50 times (the book is 300 pages) until I had EVERY WORD and EVERY SENTENCE exactly as I wanted it. If you don't want to wait that long, however, you need to hire a professional who can quickly spot where you went awry.
A good editor does not arrogantly rewrite your book and lose your personal voice. Like an expert gardener, I will eliminate the weeds and leave the overall conception of your work intact, as well as your personal writing voice.
A good editor makes CONSTRUCTIVE suggestions, and never makes gratuitous, negative comments. My goal is to enhance the positive aspects of your book and eliminate anything that detracts from a good read.
My mission is to make your book the very best it can be.
Understand that I cannot rewrite your book, and that the quality of your narrative depends solely upon what you put into it.
I love writing, however, and I will treat your book as if it were my own work. And in fact, it IS my work, because of my association with it. Your published manuscript is a reflection of my editing; and therefore, I want it to be of the highest quality.