Developmental Editing helps you fine tune the sequence and the order of your book. Have you presented the information in a format that makes sense to the reader; does each piece of information build upon previous material? If you’ve had a few people read drafts of your book and they’re confused, you need developmental editing. A developmental editor may even be able to help you uncover features in your work that will create a powerful experience for your readers.
It’s always sensible to follow due diligence in submitting your first draft of your book to an editor by making as fool-proof a copy as possible for you because first, it is your own creation, and secondly, a horrid draft sent your editor can end up frustrating both you and him or her. Make your own corrections to the extent you can, so that will make the job of your editor that much lighter and the end result would be a lot more effective.