As one editor turned writer says in an essay in The Guardian: “It is extremely dispiriting to sit in a room with your editor and be told that the book (in my case novel) you have lovingly crafted over a long period of months is not gripping, charming and perfectly turned out, that it is in fact repetitive, unconvincing, too sketchy here, too drawn-out there. But I'd be mortified were anyone other than my editor and agent to read the first draft. It is vital that an author has someone willing to be tough with them. It's in their best interests, and if that person is employed by the author themselves on a freelance basis, I question how tough they'll be willing to be.”
Read more about the importance of editors.