> Why would I use Grammarly/Superhuman for writing with LLM assistance, when I have an out-of-box alternative that, at worst, is equal?

I think the answer is basically that they have brand recognition and they're trying to ride it. Right now, they have two bad choices: become irrelevant more quickly by having a product that's inferior to built-in LLM tools, or become irrelevant more slowly by having a tool that's comparable (and also works anywhere on the internet, not just on specific websites).