Don’t we get the most value from our tests when they fail? We may wear our seatbelt every time we get in the car but when we get into an accident and use our seatbelt is when we are most grateful for having it. This is also true with tests. It is not until a test breaks that we are really thankful that we have the test.
The fact that I have tests to verify my code is working gives me confidence when I make changes to the system by automatically running regression tests. But this is just a side benefit to me, although it’s a big one. The real benefit to me is that test first forces me to write high quality code.