1. Your debug time will drop to near zero.
2. Your tests will provide examples for how every part of the system works.
3. Writing tests forces you to decouple, thereby creating a better design.

***

Ignore the above.

If you follow TDD, and use it to build a test suite that you trust. And if that test suite executes in seconds (a design goal). Then you will not be afraid of the code. You will not be afraid to clean it. You will not be afraid to fix it. You won't be afraid to do anything to the code, because your tests will tell you if you've broken it.

It is one of the great tragedies of our industry that so many programmers have decided that they can't have that test suite, or that it would be too expensive to create. They live in fear of their code. And this slows them down immeasurably.