I can give an anecdote. I needed to design a load testing system that would ramp up to an arbitrary number of users of certain types, over an arbitrary amount of time.

Rather than just make a loop and increment every so often, I made the system output it's "plan" for the user ramp, and a second component execute the ramp plan. We had a bug in the calculation which we were able to see easily by reading the "plan." Without this step, we would have been unsure if it was the calculation of when to add users, or the implementation of adding a user that had a problem.