Use cases are vital and central to the UP, which encourages use-case driven development. This implies:
Requirements are primarily recorded in use cases (the Use-Case Model); other requirements techniques (such as functions lists) are secondary, if used at all.
Use cases are an important part of iterative planning. The work of an iteration is—in part—defined by choosing some use case scenarios, or entire use cases. And use cases are a key input to estimation.
Use-case realizations drive the design. That is, the team designs collaborating objects and subsystems in order to perform or realize the use cases.
Use cases often influence the organization of user manuals.
The UP distinguishes between system and business use ...