The point is that the existing behavior is de-facto correct by virtue of being the existing behavior. The goal here is not "Prove this software meets a spec" i

The point is that the existing behavior is de-facto correct by virtue of being the existing behavior. The goal here is not "Prove this software meets a spec" it is "This software is the spec, and we must not regress". There are many more systems that act as a defacto spec than that are built to an external one. If you choose to then move to a world where you want an external spec, as I said, at that point some of these tests will be thrown away or refactored. Most systems never make it to that stage. This is especially common when you have legacy software where the person or team that wrote the code is no longer there. It takes a monstrous amount of effort to derive a meaningful spec, and step 0 is always preserving existing behavior.