You don’t want your readers to get distracted by errors in your book. So don’t let your readers find any errors — let me find them instead. You want an editor who understands the conventions of your genre. I specialize in fantasy, dystopian, paranormal, and speculative fiction. You want an editor who doesn’t try to rewrite the book in her own voice. My goal is always to retain the author’s voice — just to make it a cleaner version of that voice.