Defining what the target uses cases are for an API, while also regularly evaluating how existing consumers are using an API, is how you tighten the direction of an API. The what portion of uses cases for an API, is about focusing on the business side of what API consumers are doing with an API.
What API consumers are doing with an API should be shaping the design of the API over time. The what is being focused on business solutions an API offers to consumers, considering the retail uses of APIs in desktop, web, mobile, and device applications, but also what industry and business sector it is being applied.
Use cases help you round out what will be done with API resources and capabilities and provide more context for the number of APIs, and the shape of requests and responses. How you define what consumers will have downstream effects on performance, security, and other areas of the API lifecycle. Knowing what your API consumers are doing are how you inform your road map, inform your marketing efforts, so take the time to do it well.