Why do you need an editor? Here are two good reasons:
Reason #1: Writing is a foreign language, of which no one is a native speaker. Writing resembles thought but is different enough that translation is required. Inventing a story is usually the easy part; what writers struggle with is how to turn their story ideas into outstanding sentences and paragraphs. If you speak more than one language, you know there are multiple ways to translate any sentence; therefore, for commercial purposes, it’s essential to get a second opinion.
Reason #2: If you’re like me, over the course of multiple drafts you rewrite every sentence several times. A phrase that seems brilliant in April dissatisfies in May. You realize that the final sentences of many of your paragraphs should instead be the first sentences. Everything is plastic.
Part of this is due to your ever-improving understanding of the way that vocabulary, grammar, and rhythm work together to transmit ideas. You get smarter as you read and write, and your tastes keep changing. Ergo, when you judge and revise your own writing, the goalposts keep moving. A hired editor’s judgment is not necessarily better than your own, but his or her written opinion is a stationary landmark, and that’s easier to steer by.