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).