Unfortunately, many developers believe that tests are a waste of their time. After all, they do not write bugs — only other programmers do that. Most of us have said, at some time or other: “I would write tests if I had more time.” If you never write a bug, and if your code will never be changed in the future, then indeed tests are a waste of your time. However, this most likely also means that your application is trivial, or that it is not used by you or anyone else. Think of tests as an investment for the future: having a suite of tests will be quite useful now, but it will be extremely useful when your application, or the environment in which it executes, changes in the future.
Tests play several roles. First, they provide documentation of the functionality that they cover. Moreover, the documentation is active: watching the tests pass tells you that the documentation is up-to-date. Second, tests help developers to confirm that some changes that they have just made to a package have not broken anything else in the system — and to find the parts that break when that confidence turns out to be misplaced. Finally, writing tests at the same time as — or even before — programming forces you to think about the functionality that you want to design, and how it should appear to the client, rather than about how to implement it. By writing the tests first — before the code — you are compelled to state the context in which your functionality will run, the way it will interact with the client code, and the expected results. Your code will improve: try it.
We cannot test all aspects of any realistic application. Covering a complete application is simply impossible and should not be the goal of testing. Even with a good test suite some bugs will still creep into the application, where they can lay dormant waiting for an opportunity to damage your system. If you find that this has happened, take advantage of it! As soon as you uncover the bug, write a test that exposes it, run the test, and watch it fail. Now you can start to fix the bug: the test will tell you when you are done.