I worked on a project with no tests. We had to refactor a specific functionality changing database entities, augmenting algorithms and adding behaviour. Hundreds of possible cases, no way to be tested manually. I refactor all the code to be "testable", I wrote hundreds of tests. I wouldn't have done it without tests. In the same project, a single algorithm had a dozen of parameters, dozens of possible scenarios. An issue occured in prod. Luckily algorithm had over 60 tests, we added a red test, fixed, run all green and we were very confident of the release, same confidence not possible without tests.