How does server-side (backend) webservice integration differ from frontend integration, and what does it buy you?

In backend integration your webserver calls the external API itself and sends the finished result to the client — the browser never talks to the external service directly.

The difference is who makes the external call. In the frontend model the browser fetches the API directly; in the backend model the external call happens entirely between two servers:

Frontend:  Browser → your server (HTML) → Browser → External API → Browser
Backend:   Browser → your server → External API → your server → Browser

Routing the call through your server unlocks several things the frontend model can't do:

• API keys stay secret — the key lives on the server and never reaches the user's browser, so it can't be copied.
• Aggregation — your server can call several APIs and merge their results into one response.
• Caching — the server can cache a slow or expensive API response and reuse it for many users.
• No CORS needed — server-to-server requests aren't subject to the browser's cross-origin restrictions.

The cost is added latency: the data now makes an extra hop (browser → server → API → server → browser) instead of going straight from the browser to the API.