Here's the thing. When I'm rapidly prototyping and exploring, I'm constantly asking questions and making hypotheses about what the thing I'm exploring will do when I turn various knobs and pull different levers. Part of any good process of making guesses and comparing the expected and actual results is making notes about the test and results. Those notes could be written in natural language, and many notes should start there, but why not formalize the guesses and discoveries and write them as executable, reproducible, tests?