Even though APIs are designed to be consumed by code, the initial interaction is typically by a human. A working implementation might respond with correct results if the right parameters are passed, but without proper documentation, it can be unusable.
Under-documented APIs can reduce collaboration by different teams with your application. Often, required resources such as conceptual overviews and getting started guides are not found, leading to a frustrating developer experience.
Finally, since most web APIs are initially accessed using web browsers, an ability to interact with the API within the documentation itself is very useful. Even if the documented behavior differs from the code, the ability to try and verify the behavior ...
