A good copy editor can significantly improve a book in all sorts of subtle and beautiful ways; a bad one can slash-and-burn their way through a manuscript and render it almost unreadable.
If you've ever considered hiring a copy editor to look at your work before you send it off to publishers or agents, here's a handy list of questions to ask before you trust them with your book. Read every word. Learn it off by heart. This is why professional editors are to be respected and adored.