It accumulates the system's requirement, which includes both internal as well as external influences.
The main purpose of a use case diagram is to portray the dynamic aspect of a system. It depicts the high-level functionality of a system and also tells how the user handles a system. It models the tasks, services, and functions required by a system/subsystem of an application. It encapsulates the system's functionality by incorporating use cases, actors, and their relationships. A use case diagram is used to represent the dynamic behavior of a system.
