The target is still not quite reached when the book is written. Anyone who believes that the text can be directly printed, has never been annoyed about mistakes in a book like spelling, logic leaps, stylistic or content errors. After writing a book it needs a final spurt, which should not be underestimated. And this is almost as important as the writing itself. It is important to make the book look nice and above all error-free. This is best doen with a professional editor and proofreader.
Remember, your book competes with a huge number of regular new releases. Readers are critical and want an exciting and error-free text. In addition, you save yourself the time-consuming early reprint of your book.