In this age of automation, some would have us believe that proofreaders are at risk of replacement by automated "proofreading" services like Grammarly. You have probably had some frustrating experiences with spell check and grammar check in Microsoft Word, which should tell you all you need to know about computerised proofing: that it does not work reliably.
"Hold on," you might say. "Grammarly is more than a simple spelling and grammar checker, right?" Apparently so: Grammarly claims to be an "automated proofreader and personal grammar coach". That such a thing is available for free (or a small fee for the full package) sounds too good to be true. Well, how does that old saying go?
Language blog Grammarist wrote a revealing article on the limitations of Grammarly's services. The digital "proofreader" was no better than Word in correcting spelling and grammatical errors, and it fails more often than not to pick up on misplaced words. Highlights from the article include the following failures by Grammarly:
- It did not correct the use of "definately", one of the most common misspellings of "definitely".
- It incorrectly suggested changing "are" to "is" after a plural noun.
- It incorrectly highlighted a complete sentence as a sentence fragment (reminiscent of Word).
- It did not correct the use of an apostrophe (') in making a noun plural.
The article also notes that the plagiarism checker is unreliable and that the program tends not to pick up on differences between American and UK English.
In the interest of fairness, I looked for another source to corroborate this verdict on Grammarly. Good Content Company found that Grammarly was not only inferior to a human proofreader, but to Microsoft Word as well, missing more errors than it caught.
This should be all you need to know about so-called proofreading tools. There is no substitute for a real proofreader, nor is there likely to be in the foreseeable future. 
This is not to say, however, that Grammarly is completely useless. Running your text through Grammarly's free spelling and grammar checker, and correcting some of the most obvious errors, could make life easier for your proofreader. You should, however, take their suggestions with a grain of salt: if they sound wrong, they probably are.