I use it as a proofreader, not to improve my writing. It’s difficult to proofread your own work, and Grammarly is a useful assistant. Plus, I’m British and I often write on behalf of American clients. I’m pretty good at following US English standards because I’ve been doing it for a long time, but the odd Britishism slips through and Grammarly usually catches it (although a standard spell checker would too, I suppose).