Tests demonstrate the current behavior of the software. If you don't have tests then all you'd need to do is manually run through millions of test cases manuall

Tests demonstrate the current behavior of the software. If you don't have tests then all you'd need to do is manually run through millions of test cases manually, hardly better. What you've done is be the first person to go through a test case. Once there's a test that behavior will be documented, which I would much prefer to having to wait yet again for someone to find this test case.