What is the primary purpose of a use case diagram?

The primary purpose of a use case diagram is to illustrate interactions between users (or "actors") and the system being designed. Use case diagrams focus on capturing the functional requirements of a system by showcasing how various users will interact with the different functionalities. Each use case represents a specific goal or task that an actor wishes to accomplish within the system, providing a clear and visual representation of the system's intended functionality from the user's perspective.

This type of diagram helps stakeholders understand the key interactions without getting bogged down in technical details. It serves as a communication tool among developers, business analysts, and other stakeholders, ensuring everyone has a shared understanding of how the system is expected to behave in relation to its users. This emphasis on user interaction sets use case diagrams apart from other types of diagrams that may focus on architecture or static models.