Why do I need an editor to edit my book?
An editor will help to polish the gem that is your book. You wouldn't want your book to fail because of easy to spot errors, loose ends or story points that just don't add up. A good editor will help you correct everything from grammatical errors to overall organization and can point out and help you fix anything that impedes the smooth flow of ideas from you to your reader.
The reading public is intolerant of things that make a book difficult for them to read. No one wants to stumble through a story that has lots of flow issues and they certainly don't want to be distracted by poor grammar and typographical errors. If readers and reviewers find enough errors, your book is doomed to failure no matter how hard or long you worked on it or how beautiful the printed product looks.
Your editor will help you to refine ideas and make sure the reader knows exactly what's going on and gets the point or storyline exactly the way you intended. He or she will also help you to find just the right word for an awkward sentence, and can give advice on how to make a scene come to life for the reader.
This is not a job the author can do well because he or she lacks distance from the work and objectivity. It also isn't a task an author's friends or relatives can perform, unless one of them is an editor. Substantive editing is a time-consuming process which only an educated and experienced editor can do correctly.
A good editor is an author's partner in writing. He or she has the same goals as the author to make the book as good as it can possibly be. As an author, you must learn to take constructive criticism and suggestions and use them both to make your book better.
When the editing process is finished, the author can be proud of the work and be confident reviewers and readers will be judging the work, not the errors.