It is nearly impossible to edit your own work thoroughly. Even if you have a strong command of the English language and are adept at grammar and punctuation, you remain too close to your own work to see it objectively. You wrote it. You breathed it. You even dreamed about it. Being so close to your work will often blind you to what it actually says. Your mind will fill in what you mean to say, not what is really written. Your mind already knows and understands every concept you put on paper, but a reader is not a mind reader. An objective, critical editor will spot errors in logic, question confusing sentences, find lapses in grammar, erase redundancies, and point out contradictions.
Your work is a reflection of you. Many authors who skip the editing process publish their work only to spot embarrassing error after error after publication. The work and the errors becomes part of your reputation–one that is picked up on from readers all the way to publishers. You want it to be your very best, and a professional editor is essential to making it so.
Even editors often will retain the services of another editor to edit his own work. This is not a reflection of the first editor’s skill in writing or editing. It is merely the reality of the situation. It is difficult to properly edit your own work.
Another benefit to choosing an editor is the fresh perspective he can bring to your work. A new perspective is essential to an overall well-rounded work. What you assume to be easily understandable may be confusing to most readers. A good editor will pick up on problem areas like that.
Do you need an editor? In most cases, the answer is ‘yes’!