What happens when you refactor your code? It's fairly obvious what happens: you change the name of the method being called. However, the benefit here with TDD is you still retain your testing ability. More importantly, anywhere in the code also using the method can be easily discovered. Finally, by doing this, you've automatically updated your documentation as well.
So, let's see, you've updated tests, code, and documentation all in one. Whereas if you didn't have tests, you'd have to update the code through manually searching and testing everything, and still have to remember to update the documentation.
