The same way world-class athletes need coaches, even New York Times bestselling authors need editors. Stephen King and J.K. Rowling have them. It's part of the human psyche: we're too close to our own writing to see it objectively. But there are some other critical reasons why authors, especially debut authors, need editors:
~ Editors catch mistakes
Mistakes happen. Even in NYT best-selling novels. The goal is to make them happen as little as possible. You're not going to accomplish this editing your own work. And if your plan is to rely on a software program to catch what you miss, good luck with that. Those things are marginally helpful, at best.
~ Editors help brainstorm ideas
You can't think of everything, and sometimes another pair of experienced eyes is just what's needed to make the scene/chapter better. It almost feels like Christmas morning when an editor gives you an idea to finally fix that damn problematic scene you've been toiling over for week (or months, maybe years). And who doesn't love Christmas morning?
~ Editors kick us out of a rut
I can't tell you how many times I've been stuck somewhere, and talking through my issue helped me get past my writer's block. And writer's block not only sucks, it actually gets worse the longer you let it go on. That means you want to get out of it as quickly as possible. Having an editor kick you back into gear right away is critical.
~ Editors provide insight on characters
Let's say you want your heroine to be strong and edgy, so you write her doing what you perceive to be strong and edgy things. But then when you're finished, honest readers tell you she seems like a raging b*tch. You didn't mean to, but you went overboard on the "I'm a woman hear me roar" bit. I can't emphasize enough how easy this is to do. I might even go as far as to say that everyone gets something wrong with their hero/heroine the first few drafts, because it's that hard to get right.
Think of this in terms of a real life situation: you go to a party and meet a girl named Ava. Ava's a really cool person, but she just happened to get dumped the day before. Tonight, she's had too much to drink, and you find yourself in a blubbering conversation with her where she overshares and rants on about her now ex. You can't help it, but you form the opinion that she's negative and bitter. You do this because you just met her. If you'd known Ava for years, you would've known that this was just a really bad night for her, and that's not actually how she normally is.
So back to your heroine. In the first scene of your book, you've likely put her in a trying situation because you're writing a novel, and that's what novels do. Her reaction to this tough situation is being judged by a whole bunch of people who've never met your character before. You've got to walk a delicate line of hitting all the right notes to make her either likable and/or relatable, and no matter what, someone readers want to root for.
~ Editors plug plot holes
Usually, we've written so many drafts of our novel that we get what I like to call version blindness. This is where we're reviewing our novel for the umpteenth time, and we don't think twice when we see a reference to a gun that was hidden in the closet in chapter two. Except in the editing process, that chapter was cut three versions ago. Now, we're referencing a non-existent gun that isn't non-existent to us because we'd read chapter two a thousand times before it was cut. We've fallen victim to version blindness, and it happens to everyone. Editors will help us with this.
~ Editors catch lags in conflict
We're so close to our heroes and heroines that we get personally invested in their internal conflict. Again, I'll use a real life example to make this clear. Let's say you have this weird quirk about jeans. (I have this so I can pull from personal experience). I collect jeans like baseball cards, which sounds really strange (and it may be, possibly), but when you hear what's behind it, it makes more sense.
When I was young, my parents divorced and I lived with my dad. Mom would take me to the mall on weekends where we'd shop for new, fun jeans. Then when I went to college and I gained the freshman 15 (except for me, it was the freshman 30, whoops), none of my fun, cute jeans fit anymore. As a weight loss goal, I wanted to get back into all my jeans. Somehow, jeans became a reward for things in my life. Lost weight? New jeans. New job? New jeans. New boyfriend? New jeans. You catch my drift.
Alright, back to my point about your novel if you're still reading. How much did you just care about my jeans obsession? Probably not all that much. But I care about my jeans things because, hello, it's near and dear to my heart, and there's SO much history and emotion tied to it! But there's a big difference between what we care about for ourselves versus what others care about for us. You may be putting your hero/heroine through something that seems big and huge to you, but your readers are Zzzz-ing. This is where an editor can step in and tell you what no one else dares say: "Hey Terra, I know your jeans thing is a big huge deal to you, but to everyone else, eh, not so much."
This is where I go into my SHAMELESS PLUG, so you can stop reading now if you wish.
Editing is expensive. It's worth it, but expensive. The mistake I made was paying for line and copy editing before my story was solid. So all the editors' work had to be redone, which wasted time and money. I don't want others to make the mistake I made. Get your story right FIRST, and then hire copy and line editors. Chances are you have scenes you don't need, characters that aren't developed right, lags in conflict you can't see and flow issues things that have to be redone. Everyone does, even experienced authors.