You may wonder why you would want to hire an editor when you can just do the editing yourself. As a blogger, I know first-hand how perilous it can be for writers to edit their own copy. Think how many times you have received a reply to an email and noticed that your original email contained errors. It turns out that editing one’s own writing is remarkably difficult. Once they are initially missed, errors tend to become invisible to you when you are reviewing your writing; your eyes skim past them.
It is not only problems of spelling, punctuation, and grammar that can escape your notice; familiarity with your subject can result in explanations and descriptions that are incomplete or confusing to readers. An editor will ensure that your copy is intelligibly and concisely written.
You may also ask yourself why an editor is necessary when your word processing program already has spell checking capabilities. Spell check, it turns out, is only partially effective at catching problems. For example, it won't flag homonyms (e.g., council/counsel). Try typing “The school’s principle had been a teacher for many years” and see if your word processor catches the mistake. Mine doesn’t. In fact, words used nonsensically are regularly accepted by spell check programs. And, of course, these programs have nothing to say about confusing, needlessly wordy, or jargony prose.
Any manuscript requires review by a careful and experienced editor.