[cjwoodall]

Testing validates that the program is correct according to some base of assumptions. You run into this a whole lot in embedded systems, where mocking hardware is difficult. You can't feasibly unit test against real hardware (most of the time), so instead you unit test against your assumption of what the hardware does and verify you respond according to requirements.

Has the benefit of proving correctness of your assumptions, which makes it easier to debug once you insert it in system and things inevitably are not 100% right. It gives you a way to reason about what your code does, what might be different, and then allows you to revise your assumptions and get your new solution in place and tested without the often long wait times to do manual testing on deeply embedded hardware.

Sometimes traditional TDD is the answer, sometimes simulations are the answer, and sometimes you need to just get out of your chair and test it out. It is a tool!