Grammarly's business model in a nutshell, like many other scams nowadays:
1. Hire a programmer or two for a week or two.
2. Build a rudimentary web-based word processor
3. Offer free trials of course, but subscription plans must be no less than $30 a month.
4. Advertise your crap like crazy.
5. Hope for people to forget canceling their subscription at the end of the free trial (chances are some will).
6. No need for providing customer service for those whose credit card has been charged at the end of the free trial.