Tests can be useful as additional documentation. Type::Tiny::Manual::AllTypes explicitly links to a directory in the Type::Tiny test suite that is intended to act as in-depth documentation. It tests combinations of things you might not expect to ever see, so perhaps wouldn't see explicitly documented elsewhere, like "should a blessed arrayref pass the type constraint for negative integers?"

Certainly most of the test suite is not written with that in mind, but I do think having a test suite that is organized and provides a demonstration of every feature of the software is a worthy goal, and once that goal has been achieved then your tests will act as documentation whether you intended it that way or not.