If you have an application that has a lifespan of years, developers will come and go. The unit tests provide some provability that is abstracted from the develo

If you have an application that has a lifespan of years, developers will come and go. The unit tests provide some provability that is abstracted from the developers. A developer leaves, they've left behind tests that demonstrate that the code fulfills the requirements and perhaps shows how the code interacts with other areas of code. A developer arrives, they can see the requirements and how an area of the codebase works by examining unit tests. Of course, this presumes well-written tests that are actually thorough. Unit tests also give you a measure of confidence that allows a freer hand when refactoring code. You can change code all you want and if the unit tests still pass, you haven't broken things (again - if the tests do cover all the expectations). Some level of sanity is required around test-first, test once the code is written, how many tests you need, what level of depth, etc. A one-off app to fulfill a quick-and-dirty need doesn't need as much testing as a core platform that will be running business critical code for years.