In the earliest stages of your manuscript, you have a lot of things to think about. The development of characters and story lines can be very time-consuming, and getting the whole story developed and written correctly can be a monumental task. By the time you've read, re-read and read it again, details can begin to blur.
A good book editor will not only check grammar, spelling and sentence structure, they'll make sure that your characters and storylines follow a logical path; they'll also make sure that your story doesn't suffer from 'soap opera-itis' - when I was younger, I always laughed when a soap opera character had a baby, after having had a hysterectomy two years earlier. While I'm sure you won't make such glaring oversights, you will want a clear pair of eyes to check your story for bumps that be distracting.
While your book is being edited, it should also formatted correctly, so it's ready to put into a page layout program; any unnecessary formating should be removed before it's laid out for printing, so all the pages have a uniform look.
If you'd like to discuss your project with me, please email me with contact information and I'll get back with you as soon as possible.