(Source: University of Cambridge)
So many people ask these questions. Why hire someone to proofread? What do they actually do? I can just use Grammarly to check it all over! Well, you'd be surprised at the work we actually do.
When we go through training, we are taught to read things letter by letter. When reading for a client, we don't see a full word straight away – we see each individual character and space on the document in front of us. The above picture is a great example of how our brain can read something without the letters being in the correct order. When you've read something numerous times (like I'm sure you'll have read your own writing many times) your brain will skim over any mistakes if the first and last letters are in the correct position.
You could have a really great eye for detail, and be great at proofreading your own work, but before you send it off to print your cat could sit on your keyboard and add a quick 'lfvbjelrgb' without you realising. A proofreader is basically a back-stop, to make sure nothing slips through the cracks.
Another thing that proofreaders check for are things called widows and orphans. A widow (in literary terms) is a paragraph-ending line that falls at the top of a new page/column, meaning it's separated from the rest of the text. While this may not seem that important, it can distract a reader and is best to avoid if at all possible. Likewise, an orphan is a paragraph-opening line that sits on its own at the bottom of a page before the rest of the text on a following page.
"An orphan has no past; a widow has no future." - Robert Bringhurst
Does anyone notice a double space? It's not always obvious, especially when using MS Word. There are many times when the beginning of a sentence has an extra space that needs to be removed – the proofreader is looking for something that isn't even physically visible!
It isn't just about spelling errors either; although that is a big part of the job. A proofreader will check for consistent punctuation, like if you're using single or double quotation marks, and if they're consistent throughout? If you've put the punctuation in the correct place within those quotations? Have you used a hyphen instead of an en dash? Have you correctly used apostrophes throughout? Are all numbers mentioned written out in full or do you use digits, and if it's one or the other is it consistent all the way through? There is so much to it. I promise you.
Not everything is going to be picked up on by the spell-check or Grammarly. A computer, unfortunately, cannot fully comprehend the complexity of the English language.
It basically boils down to how much of a perfectionist you are. If you want your work to be perfect, and to create the best possible impression, I'd recommend hiring a professional to check through your work. At the end of the day, you get what you pay for.