One of the most pleasurable benefits of TDD is that I can refactor code and not worry that I'm going to break something. On larger projects this is really liberating, I can develop with the same freedom that I did on day 1. It means that a code base can evolve smoothly and naturally, assimilating new requirements. Without that safety net you end up with code that gradually becomes a poorer fit for the requirements, changes becoming huge snaps as they are delayed over the fear of regressions.
There's 2 aspects to it. First, the coverage which gives you the safety net. Second, specifying behaviour before implementation which allows you to maintain tight alignment between your requirements and code as they evolve. According to the BDD guys it's the second aspect that is more important.
