while agreeing with almost all points here (hackernews never let me down), I would like to point out that tests give specially newcomers the confidence to start

while agreeing with almost all points here (hackernews never let me down), I would like to point out that tests give specially newcomers the confidence to start to contribute as soon as the development environment is up and running. So yes tests not only helps us avoiding regressions, but it ramps up dramatically the on-boarding of new developers.