This is an excerpt from CreateSpace, my on-demand publisher. They offer editing services but regardless of who you use, my experience has shown the value of a professional editor.
1) Immersion: Readers lose focus and can't absorb your writing if they're distracted by grammar, punctuation, and typographical errors.
2) Professionalism: You get one chance to make a first impression. How will readers--who are now reviewers--perceive your book if you haven't had an objective, second pair of eyes polish it before publication?
3) Efficiency: You save time and money in the long run by having a professional editor review your manuscript upfront. Imagine this: You receive a review critical of your manuscript's editorial quality. Now, instead of focusing your efforts on marketing and selling your book, you have to invest even more time and money in editing and reformatting services.