With TDD, the inner programming loop is:

1. form a belief about requirements

2. write a test to express that belief

3. write code to make that test pass

Without TDD, the loop is:

1. form a belief about requirements

2. write code to express that belief

3. futz around with manual testing, REPLs, and after-the-fact testing until you're sufficiently happy that the code actually does express that belief

And in my experience, the former loop is faster at producing working code.