Uses cases provide a simple way to align the design, development, and delivery of APIs with business interests, but most importantly customer use cases. Use cases are about pausing and putting some thought into how APIs should and should not be used, and documenting things over time.
Defining uses cases focuses on the simple who, what how, and why of API consumption. Use cases are being honest about the value being provided by operating APIs, and developing an awareness of what consumers needing, then aligning this with the business objectives behind producing an API. There are other approaches to aligning API operations with business operations, but use cases provide a simple way to start.
Use cases can define actual customers or as part of a more generalized demographic, depending on what is available. Once defined, use cases should be used to align with all available operations available in the technical contract, associating operations with each use case. The goal is to reduce the drift that occurs across the API lifecycle, keeping product and engineering aligned at every turn.