Some examples would be:

1. Alice is the controller of a machine in a factory, Bob is a remote worker controlling the machine from home. (there could be thousands of Alice, one Bob)

2. Alice is an internal system in Enterprise Data Center 1, Bob is a Microsevice in the Cloud

3. Alice is health monitoring device, Bob is a remote doctor's tablet. (there could 100s of thousands of Alice. On of Bob)

4. Alice is a connected car, Bob is a service in the cloud, that uses a third party vendor's managed Kafka to communicate with Alice and wants to guarantee secure software updates / machine learning model updates to Alice (there is a fleet of Alice).

etc.

Any use case where you're using third party services/intermediaries and want to keep the vulnerability surface of your application as small as possible.

It is right at the top of the page. Lack of trust in the intermediaries. Which is often the correct level of trust.

Yes it is very well described, as a meditation on architecture. But I still don't see what real world use cases are.