Reading your work out loud benefits
According to Duke University, among many other respected organizations and sources, reading your work out loud isn't just something for 4th graders learning to write. "Our mouths typically work much slower than our brains; as a result, reading aloud forces the brain to slow down and examine the piece of writing more carefully and from a different perspective." If you ever typed "the the" and NOT caught it on a read over, and then had someone else catch it and read it back to you, you know this is true!
While "another pair of eyes" is a necessary step in the process, self editing can save you a lot of waiting time, and money. If your sentences all contain multiple errors in the basics, it's harder to hear your voice - even for an editor. They have to pull back and try to see the big picture.
You can train yourself to catch many of the more mundane, small errors so that your editor can focus on the harder work. Typos, repeated words, spelling errors marked by your word processing software, misspelled character names, and missed words should all stand out when you read your work aloud.
Many people write something, and get excited to move on - but they don't realize that the very next step is to read your own work over, not once, not twice, but 3 to 5 times. That takes time, and effort - and if you can afford to pay someone else for the dozens of hours it takes, then go for it! But if you'd rather scrub and polish your own baby before you hand it off, you'll feel all the more accomplished. When you think you are done, you should be able to do an entire read through without having to correct any mistakes. (Ok, you might find something - but one mistake in 180 pages, not 180 mistakes or 360.)
Read it sentence by sentence up the page instead of down - read it to your dog - read it to your 9 month old - read it digitally, read it printed out - read it with a red pen in hand - read it with rainbow pens - and remember to own your work, you have to both write it, and read it!