I agree with you about planning, but "test driven development" isn't haphazard; it's not just clicking around to see what happens. It's a way of formally specifying your design that simultaneously confirms that your code does what it should.

For instance, you write a test that says the user should get a "not authorized" if they try to access another user's account. You run the test, and it fails. You change your code to make it pass. And every time you update your app from then on, you re-run the test and make sure you didn't make that test fail.

Having a design on paper isn't as useful as having one that you can PROVE your code conforms to.