SSR is used for pages as well, but prerendering means that rendering happens at build time instead of when a visitor visits the page.

This function allows you to modify (or replace) a fetch request for an external resource that happens inside a load function that runs on the server (or during pre-rendering). For example, your load function might make a request to a public URL like https://api.yourapp.com when the user performs a client-side navigation to the respective page, but during SSR it might make sense to hit the API directly (bypassing whatever proxies and load balancers sit between it and the public internet).

When fetch runs on the server, the resulting response will be serialized and inlined into the rendered HTML. This allows the subsequent client-side load to access identical data immediately without an additional network request.