Basically you get a PWA for free with your app. All "native" features that you have can then use web device apis if they exist.
Basically React Native Web allows you to have a lite version of your app that lives on the web.
That way if you preferred Vue, Angular, Ember, Elm, etc you could use that.
You can use something like Solid or JSX lite to maintain standard components that are compatible with most of those frameworks and web components.
Even if you use web components that doesn’t change the fact that web apps don’t have access to native apis without compilation.
Basically you get a PWA for free with your app. All "native" features that you have can then use web device apis if they exist.
Basically React Native Web allows you to have a lite version of your app that lives on the web.