In my previous job I was maintaining a big project which originally had zero unit-tests, it was apparently working well but once I started adding TDD I found several security issues, edge cases, and memory overflows; by the end of my contract I had written more than +2,800 unit and integration tests and the code coverage was still low at around 60% but I am very confident that new developers are going to take ownership of the project from the beginning without much training just because I wrote those tests. I consider testing an useful way to allow new contributors to join the development process because they will not be afraid to touch the code because the tests will throw a warning when something breaks after any modification, without tests they would surely have to spend more time checking the details of their modifications more than integrating into the team.