Tests ensure you always have two consumers for an API (the API "in-place" as used by the functional portion of your system, as well as "how the tests use it").

Tests ensure you always have two consumers for an API (the API "in-place" as used by the functional portion of your system, as well as "how the tests use it"). This ensures new uses of the API have a path/model to follow and prevents you from having to "instantiate the world" just to call your particular function. I also think of tests as "scaffolding" when building a skyscraper. They make it easier to access certain "things" before they're fully complete and are a cost that makes building the actually building less risky and quicker. Why does no-one question the value of paying for putting up and tearing down a scaffold when building or repairing a building, but can't see the value of unit testing?