Proofreading is entirely different from editing, especially heavy editing (developmental or line-editing). It does not contribute towards the final product whereas the latter not only improves the content significantly but makes the manuscript the best readable version of itself.
Proofreading corrects minute errors, typos, spelling errors, textual inconsistencies and punctuation. It does not leave any room for improvement or up-gradation of your Art. And more importantly, the final manuscript will more likely look similar to what it was prior to getting proofread. Whereas, developmental editing work on your plot, its characters and their personalities to check for major inconsistencies. How your story unfolds, how you start and end your manuscript, are you able to hold their attention midway, all lies in the domain of your developmental editor.
That does not downright rule out its importance. It is highly important to get an edited manuscript proofread before the final publishing to avoid the mishap of 2000 copies in print of ‘City Sage’ instead of ‘City Saga’.