So does hiring a person or tests which rely on entropy because exhaustive testing is infeasible. If you can wrangle the randomness (each has different ways of going about that) then you end up with very useful tests in all 3 scenarios, but only automated tests scale to running every commit. You probably still want the non-automated tests per release or something as well if you can, depending what you're doing, but you don't necessarily want only invariant tests in either case.