Why do we write tests? The most obvious reason is to see whether our code works. In the absence of a framework, we often do this manually at the time we’re adding a feature or while fixing a bug. But testing can do more for us than that.
Good tests also serve as a form of documentation. Hopefully your functions are well named with clear parameter names, indicating their purpose and form, but having tests where someone can look at exactly how your function gets called is about as clear as it can get.