The traditional description of an editor is the writer’s “midwife”: they help deliver the book, and ease the labour pains. They’re the person who picked your book out of a submissions pile and thought “Wow! This is special! I want to publish this!”. From there, they have to persuade all the other people they work with that buying your book is a smart move. So they want your book to succeed almost as much as you do. This inevitably means making some changes to it.
Together, you and the editor will find ways to improve your book’s structure, characterisation and style. You will have to rewrite bits, add others, and cut some out. Writing is such a personal thing it’s hard not to feel aggrieved if somebody asks you to change something you spent so much time and energy perfecting. But you know what? 98% of the time, that change will be right. Editors have an emotional distance from the manuscript that a writer doesn’t. They also have specialist business knowledge of the readership and market conditions. Working with a supportive, creative editor can be one of the most rewarding parts of writing a book.