Proofreading is the reading of a galley proof or an electronic copy of a publication to detect and correct production errors of text or art. Proofreaders are expected to be consistently accurate by default because they occupy the last stage of typographic production before publication.
To read the entire (lengthy) description, go to : https://en.wikipedia.org/wiki/Proofreading
So, for those authors, would-be writers and amateurs who are passionate about their work and are keen to have it published, whether in hard copy or an eBook or anything else that is available, employing a proofreader is essential. Of paramount importance is correctness - grammar, punctuation, plausibility and the elimination of 'unnecessary' words. Formatting is another key to ensuring your writing is set out in a clear and clean manner, and above all, is easy to read.
Wishing you all the best with your next 'best selling' novel!