When I worked at the Houston office of the Ogilvy ad adgency, we had a full-time proofreader. It was an amazing experience, but they don't fit into most business models. Usually, proofreading is something employees do for one another.
For small business owners like myself, proofreading is something my customers and prospects do! Painful but true. Have pity.
Moz: Google-leaked Dos and Don'ts, 2014-Dec-5 by Josh Bachynski
You want to make sure that the spelling and grammar has been checked, that all the pages have been checked for errors, that there are no miscellaneous 500 errors, there are no naked Apache 404 errors. You want to make sure that, essentially, the website has been looked over and proofed. This is, as Google tells us, a direct signal that they're looking for in their quality algorithms. It makes sense because you don't want to see a site that has these kinds of errors on it.