Use Case
A technique for capturing potential requirements of a new system or software change. It describes how a user interacts with the system to achieve a specific goal.
Impact
Use cases ensure the system supports real user workflows.
Weinto take
We prefer User Journeys over Use Cases. A use case is atomic ("Login"); a journey is holistic ("Login, find product, buy, receive email"). We design for the journey to ensure consistency across the entire experience.