Which UML diagram type helps in understanding system requirements and use cases?

Use Case Diagrams are instrumental in understanding system requirements and use cases because they visually capture the interactions between users (or "actors") and the system. These diagrams focus on the functional requirements of the system by illustrating how different users engage with various functionalities. Each use case represents a distinct function or action that the system can perform in response to user demands.

By depicting these interactions, Use Case Diagrams help stakeholders identify what the system needs to do from a user's perspective, thereby clarifying the requirements and ensuring all necessary functionalities are accounted for. They serve as a bridge between stakeholders, such as project managers and developers, by providing a clear, communicative visual that encapsulates the user experience, which is crucial for successful system design and implementation.