That's the gap server components fill. With SSR you always had the option to just use React on the server and send HTML back, no JS or hydration needed. But if you then want a single widget on the page you can go back to the old (not saying bad) days where you include some clientside js that attaches event handlers and changes some HTML. Or you mark that one component with "use client" and you're done and can seamlessly to your declarative rendering on the client and use the same components you already have.
The RSC part is mostly the streaming of the JSX result, but this only solves the issue that the market wants to work in a MPA way with the routing benefits of a SPA.
Again, nothing new, Turbolinks existed and replaced sections of your HTML with XHR, but you had to slice it up yourself and manage interactions again if the section you changed required JS. RSC takes away the latter part and you're only required to define the boundaries. That's the reason why RSC has it's place, you're able to mix and match it within the same codebase instead of maintaining one codebase for your marketing pages and one codebase for your application (and one for your docs)
