As a non native English speaker/writer there are a bunch of errors I miss, no matter how much attention I pay and how much I proofread, and these tools are useful to catch those.

I'm a lawyer. I write 10s of pages of text every day. "Reading through it once yourself" is obviously an imperfect solution. See, e.g., Poisson statistics. It's also slow and I bill in 6-minute increments. There is significant value in a grammar tool that protects confidentiality and is more effective than my wetware.

People are bad at proofreading their own work. Professional writers often use third-party copy editors and proofreaders for that reason.

I know for example David Sparks (MacSparky https://www.macsparky.com ) uses it (or at leased used it). And he was an American lawyer and he says writing has been his passion his whole life so I assume his English is better than the average person.

I use it (well, languagetool) in the free version for comments on sites like this. It directly catches mistakes I make, that I'd normally only catch on re-reads. From typos, over my brain doing weird stuff, to sometimes things I simply didn't (actively) know.

Have you considered that some people aren’t 100% fluent in English but still competent?

I’m a production editor at an uni press, and I can tell you there’s not a strong correlation between professional/academic level and writing well.