Why online? I mean you can use Libre Office.

or Pages if you are on macOS

That's a good point those are good options. Sometimes if you're already in a browser its easier to just stay there, ctrl/cmd-t "spell check" and then just paste is less actions than opening pages, selecting document template type -> create new. On macOS textedit is also a good option, might use that from now on.

There are CLI tools like aspell or hunspell that GUI tools are based on.

How you'd integrate them with your workflow is beyond me, though :)

I think many browsers have spell check dont they? Couldn't you just leave a tab with a text entry form open and type into that? Or just type the word into Google.

Maybe I'm not understanding the use case

Grammarly is nice (as a non native English speaker)