There are plenty of ways to improve your English without grammarly.
I mean I guess you can learn an instrument by reading sheet music and sheer force of will but I think most people prefer lessons.
I disagree. Grammarly has bombarded the internet and YouTube with so many ads, there are ads that make fun of Grammarly ads.
Nevertheless, people don't use it because it offers something that others don't. They use it because they don't know any better.
Even in this discussion, most suggestions for "better" is completely useless advice of "learn to write".