A Second Set of Eyes
IS 
Priceless

The Editing Phase is essential for authors to tighten manuscripts since they and software checks typically miss a number of errors... leaving them horrified if actually printed. IE:

Ever have chocolate mouse for desert...
when you meant: mousse for dessert?

An author's tired familiarity from constant rework causes eyes to pass over common mistakes, typos, punctuation, grammatical errors, awkward phrases, plus misspelled, missing, or misused words and so on.

Make your Manuscript the Best it Can Be

Revision is not a malevolent word, but one of refinement. So if you're serious about your work, using a professional editor would not only catch the more obvious errors, but solid feedback is likely to help improve pace, plots, imagery, characters, logical flow, dialog, effective scenes, etc... perhaps even enhance your story's style and reader impact. After a book has been edited, we require the author to go through their book, (reading every word) accepting (or not) changes suggested, and making any final revisions before signing off that their manuscript is the way they want it.

The Different Stages of Editing
PROOF-READ Editing:  
The least comprehensive "fast read" normally consisting of a spell check and such items as: typos, misused words (its-it's, hear-here, there-their); consistent font, page layout, and style; misplaced quotation marks, and other common mistakes.

THOROUGH Edit: 
Everything a proofread entails, plus a more in-depth procedure with respect to addressing syntax issues, verb tenses and transitions, grammar and literary technique, word choice, sentence structure, rework paragraphs and awkward phrases, et al

DEEP Edit: 
A comprehensive professional editorial review and re-writes which includes editing types above plus evaluating a story's structure; plot-holes and lost characters; illogical time, event, location flaws, and any number of other inconsistencies to improve clarity of writing, etc. Subject to negotiation, Deep Edits average between  $1500-2000 for a full length novel under 350 pages. (8.5 x 11, dbl-spaced, 12 pt font).