TDD: What's the Point?

What's the point of TDD?  This might sound outrageous, but IMHO, the point of TDD is not to exhaustively test your code as your developing it.  It's not to test every corner case, every possible input/output scenario imaginable.  It's not even -- gulp -- to achieve 100% code coverage. 

What?!  Blasphemy! 

Yeah, yeah.  100% coverage is a worthy goal, and if you're doing TDD right, you'll get pretty close anyway.  But still, 100% coverage isn't the point.

The point of TDD is to force you to design and write your code from an outside-in perspective, rather than an inside-out. It therefore also ensures that it is testable, well modularized, free of inappropriate foreign entanglements, and is most likely to work the way you need it to.  The focus is on the use of the class, not the implementation of the class. 

When you're not doing TDD, you're thinking more (sometimes exclusively) about what the class needs, and how it will do what it will do.  When you do TDD, you think more about how the class is used, what it's user's need, and what the rules are.

You write better code that way.  So TDD is a way of getting yourself to write better code.

So TDD is an approach to development -- not an additional task added to development.  It isn't overhead.  That brings me to the next topic.