What is the purpose of a use case diagram?

A use case diagram serves to visually represent the interactions between users, or "actors," and a system, illustrating how different users can achieve specific goals through their interactions with the program. It helps to identify the functionalities the system must provide and emphasizes what users can do with the program. This type of diagram is key in requirements gathering and clarifies the relationships between different actors and the use cases they can perform, thus aiding in understanding and modeling user requirements in software development.

The other options describe different aspects of software development. The description of data used in the program is typically captured in data flow diagrams or entity-relationship diagrams. The flow of algorithms is usually illustrated through flowcharts or sequence diagrams, which focus on the logic and sequence of operations. Interactions between program components might be showcased using component diagrams or class diagrams, emphasizing system architecture rather than user interaction.