Server Components is not really related to SSR.

I like to think of Server Components as componentized BFF ("backend for frontend") layer. Each piece of UI has some associated "API" with it (whether REST endpoints, GraphQL, RPC, or what have you). Server Components let you express the dependency between the "backend piece" and the "frontend piece" as an import, instead of as a `fetch` (client calling server) or a <script> (server calling client). You can still have an API layer of course, but this gives you a syntactical way to express that there's a piece of backend that prepares data for this piece of frontend.

This resolves tensions between evolving both sides: the each piece of backend always prepares the exact data the corresponding piece of frontend needs because they're literally bound by a function call (or rather JSX). This also lets you load data as granularly as you want without blocking (very nice when you have a low-latency data layer).

Of course you can still have a traditional REST API if you want. But you can also have UI-specific server computation in the middle. There's inherent tension between the data needed to display the UI (a view model) and the way the data is stored (database model); RSC gives you a place to put UI-specific logic that should execute on the backend but keeps composability benefits of components.