It is common knowledge that writing tests and having a large number of automated tests are a Good Thing.
However, without TDD, it often just becomes tedious. People write tests, and then leave it, and the tests do not get updated as they should, nor do new features get tested as often as they should either.
A big part of this is because the code has become a pain to test - TDD will influence your design so that it is much easier to test. Because you've used TDD, you have a good number of tests, which makes it much easier to find regressions whenever your code or requirements change, simplifying debugging drammatically, causing an appreciation of good TDD and encouraging more tests to be written when changes are needed - and we're back to the start of the cycle.
