As much as I think you wish this to be true, a professional edit brings, well, professionalism and consistency and industry standards to your book manuscript. A professional edit is essential, as is a professional cover design. The English teacher next door or the so-called editor on Fiverr (check credentials and experience) are often not skilled book editors. Nor is Grammarly. My book, Cover to Cover: What First-Time Authors Need to Know about Editing, explains levels of edit, cost, how to find the right editor, and why you need one. Critical readers will eat your lunch with one-star Amazon reviews if your book appears at all unpolished.