There has been a lot of discussion across the internet about why someone would and would not hire an editor. Most of the why-not's come down to the topic of costs. Editors can charge anywhere from $0.23 to $0.35 per word, so for a 100K word book that is roughly $23K - $35K, certainly a large sum of money.
Most publishing houses have a go-to editor company and will pay for the edits to an approved manuscript. But for those interested in self-publishing, like myself, it is simply out of the question.
The other reason many people say editors are not needed is because they believe they are good enough to edit it themselves. After all, they wrote it and know exactly how it should sound. Or, they've had their parents, or their best friend, or their old college roommate review it and he/she/they provided comments and corrections and the manuscript was fixed and updated and now it's ready to go. So there's no need for an editor.
Except, that isn't true. An editor is always vital because an editor can look at a document impartially and decide what does and does not work. They will also evaluate sentence structure, grammar, punctuation, and everything else. At least a really good editor will. They will deliver you back comments and suggestions, you will fix the manuscript and lo-and-behold, the book is much better for it!
Think of it another way. You would never ask your spouse "what do you think of this outfit?" because chances are you wouldn't get an honest answer. Or you wouldn't truly believe the answer because they're bias. You would seek outside advice, maybe even a fashion consultant if you wanted to go that far. You would seek help from someone who did not have any direct connection to you and would therefore tell the unequivocable truth. That's what an editor is: someone independent of you and your relations, and who can look at your book honestly and tell you what's wrong with it (and hopefully what's right).
Bear in mind, an editor is not your spell-checker. Sure they can do punctuation fixes and grammar checks, but that doesn't remove the need for you as a writer to review it, over and over again. Try reading it aloud, because the pacing will sound different in your head. Then stick it in a drawer and forget about it for a week, a month, six months. And when you're in the middle of spring cleaning and you decide to empty the drawer, you discover the manuscript and reread it. Then you'll find new things to fix you hadn't noticed before, because you will have forgotten all about the story and be looking at it with a fresh pair of eyes (like an editor does!).
Remember, you're trying to develop the best story possible so why the rush? Make sure it's perfect before handing it off to the editor so they don't have to spend as much time reviewing it. It'll save them time which saves you money.
So getting back to the topic of why you should hire an editor, the short answer is because your book will be improved, provided you find the right editor. Finding the right editor is a topic for another time, but suffice it to say, I am working with theauthormarket.com and they have found me a great editor for a fraction of the price of other companies. Once more, they are willing to work within my budget and financial availability (I am a poor self-publisher after all).
I know it's a shameless plug but after doing my research, these guys are the best deal in town. So check them out or search around for an editor that works for you. Make sure the editor is actually familiar with the genre you write or else you're liable to have the wrong things reviewed/deleted ("What's a guild?"). Either way, do your book a favor and get a professional to take a look at it. You'll be glad you did.