I find that fascinating, because interacting with the tests in our codebase (both Python and JS) answers a _lot_ about "how is this meant to work", or "why do we have this". I won't say I do test-driven development, at least not very rigorously, but any time I am trying to make a small change in a thing I'm not 100% familiar with, it's been helpful to have tests that cover those edge cases. :)