> "Before launching an API, you should come up with 5-10 compelling usecases that someone couldn't use your website for."
Not necessarily. Sometimes what you want is to combine data from multiple sources in interesting ways. A single API doesn't need to be versatile, it just needs to make useful information available.
Sure, this counts as a compelling usecase. Maybe your API just lets users list Items chronologically since that makes sense for your site, but someone else wants to get Items by location.
Many times, the only way to do what you want with data is to use the API to get everything, then transform it locally. This is obviously really inefficient (and sometimes impossible).
Not necessarily. Sometimes what you want is to combine data from multiple sources in interesting ways. A single API doesn't need to be versatile, it just needs to make useful information available.