After the book has gone through copyediting, developmental editing, and formatting, it gets a final check for errors. Sometimes the formatting process may inadvertently introduce a small error, such as a missing word. A rare typo or punctuation error might have slipped through the copyediting process. Proofreading does not introduce large changes. Remember, this happens after the book is turned into a PDF, MOBI, or EPUB file, so it is not the ideal time to do rewrites. Proofreading polishes your manuscript like fine silver, so it arrives at the publisher looking professional.