As a content writer, I began using Grammarly to perfect my article submissions. After all, it is very embarrassing to send in an edit request, when you notice a couple weeks after publishing, that you wrote "to" instead of "too". The site is very effective for perfecting my work. I find it most useful when I write late at night as that is when I make the most errors.
As a homeschool parent, and somewhat of an advisor, Grammarly helps me to catch spelling and grammar issues that I might not see at a glance. You can use some features of the site for free, but a subscription will help you find not only spelling issues, but problems with conjunctions, adjectives, adverbs, subject-verb agreement, modal verbs, sentence punctuation, sentence structure, and commonly confused words.
Useful to both content writers and students is the plagiarism checker. It will help you to prevent inadvertently using another's information in your work, and will also eliminate accusations that you attempted the act of plagiarism.
Another thing about this tool that needs pointing out is that you choose what kind of editing you want. When I am writing for Yahoo, for instance, I use the "Business" option, and when I am checking a 13 year old's term paper, I use the academic option. For my daughter, who likes to get creative, there is even a "creative option".