Importance of use case diagram

Why the use case diagram is important

A use case diagram shows the actors outside the system and the functions (use cases) the system offers them. It is important because:

1. It captures functional requirements from the user's viewpoint, in a form that customers and non-technical people can understand.
2. It fixes the scope and boundary of the system (what is inside, what is outside).
3. It is the starting point of the whole OO process: classes, sequence diagrams, test cases and user manuals are derived from the use cases (use case driven development).
4. It helps in planning and prioritising: iterations are planned by use cases.
5. It serves as a contract between customer and developers and a base for acceptance testing.
6. It is simple, with only few symbols (actor, use case, association, include, extend, generalization).

Model use case diagram: Online Library

              +-----------------------------+
              |      Online Library System  |
 [Member] ----|-( Search Book )             |
 [Member] ----|-( Reserve Book )            |
 [Member] ----|-( Renew Book )              |
              |       |include              |
              |       v                     |
              |  ( Login )                  |
 [Librarian] -|-( Issue Book )              |
 [Librarian] -|-( Return Book )             |
              |     ^                       |
              |     |extend                 |
              |  ( Charge Fine )            |
              +-----------------------------+

Scenario: A member logs in and searches a book; if unavailable, he reserves it. The librarian issues and receives the book; if the return is late, the optional use case Charge Fine extends Return Book. Renew Book includes Login.

The diagram thus gives a single clear picture of who uses the library system and what the system provides.