In marketing, a case study is an in-depth study of the effectiveness of a certain tool, tactic, or strategy. It focuses on measurable outcomes, like an increase in sales, visitors, or production hours. Also remember, people buy the same product for different reasons,  and thus use cases can vary depending on specific circumstances.
The three basic elements that make up a use case are: Actors, the system and the goal.
Other additional elements you should consider when writing a use case include: Stakeholders, or anybody with an interest or investment in how the system performs.