TDD is one of the best practices of Agile – XP, which helps reduce / detect defects early in the project life cycle, thus reducing the cost of rework.
Benefits of Test Driven Design / Development would be derived if the implementation is effective with the use of Automated Unit testing tools.
Automated Unit testing tools helps to reduce the cycle time for every refactoring / changes to requirements.
Best Practices suggest that Automated scripts for unit testing are written for requirements that are quite anticipated to change and manual testing is suggested for requirements that are stable. It is only a recommended practice, but a cautious judgement needs to be taken based on the situational occurences