I’ve been a professional writer for 30 years, writing wire-service sports stories. That’s intense deadline pressure, and I got by without Grammarly for years.
I use it on some of the freelance work I’ve been doing during the pandemic, though. It helps me catch typos (I have a tremor in my hands) and will occasionally point out a clumsy sentence. I discard a lot of the suggestions, but when I’m writing for several different clients on the same day, it is useful as a safety net.