Why is an URL set with the application service?

[railsmechanic]

I'm currently testing a large mirgation from RocketChat to Matrix and I wonder why url: 'http://127.0.0.1:1234' is set within app-service.example.yaml?

While the migration is running, I receive the following error every 5 minutes:

Error sending request to  PUT http://127.0.0.1:1234/_matrix/app/v1/transactions/1: ConnectionRefusedError Connection refused

What is the URL all about? The Application Service API documentation says that one can also use null. Please have a look at: https://spec.matrix.org/v1.11/application-service-api/

With the url set, a number of requests are cached in the database, which are then periodically executed again and again.

Can you explain this to me? Many thanks for this.