Why do web applications integrate external webservices instead of building everything themselves?

Because someone else already runs the complex system you need — so you call their webservice for the data or function rather than rebuilding it yourself.

A webservice is a machine-to-machine interface a provider exposes over the internet: you send it an HTTP request and it sends back structured data (usually JSON or XML). The point is reuse — most of what an app needs already exists somewhere as a service.

Take an online travel agency that wants to find the cheapest trip to a destination. It does not own any flights, hotels, or rental cars. Instead it plugs into existing providers — flights from airlines like Lufthansa, rental cars from Hertz, books from Amazon — and stitches their offers together. Building and maintaining a global flight-pricing system yourself would be absurd when the airlines already publish one.

The same logic drives most integrations:

• Weather — show forecasts from a weather service instead of running your own meteorology.
• Payments — let Stripe or PayPal handle card data and fraud rather than touching it yourself.
• Maps — embed Google Maps or OpenStreetMap instead of mapping the planet.
• SMS / email — hand notifications to a specialised delivery service.

Tip: The trade-off is dependency — when their service is down or changes, your app feels it. That is why a stable, well-defined interface (and good error handling) matters.

Go deeper:

• Web service (Wikipedia) — the general concept: machine-to-machine communication over the internet using XML/JSON.
• REST (Wikipedia) — the architectural style most modern public APIs (including transport.opendata.ch) follow.