Use case diagrams define the scope of a system. But use cases can also help you to design your product development organization.
Here's a use case diagram, showing the use cases of an online flight booking system:
As you can see, the diagram shows the goals that users have: an end customer wants to book a flight. The diagram does not show the steps to get to the goal, like searching for a flight.
An agile team should deliver value, independent of other teams. A goal-level use case is such a unit of value. When agile teams are responsible for entire use cases, the need for communication with other teams is limited.
The use case Book flight, for example, is valuable on its own. It ties to the users' need to get from A to B. That's why it is a selling point for the software.