I used to argue that tests were a waste of time and provided no real value. Then I got a client who insisted on having them, so I had to write them, and I started discovering subtle bugs in my code (an uncaught Promise rejection here, a reference I thought was a copy there). They weren't things that would show up in any typical manual testing or even everyday usage, but they were there and potentially could have caused problems. Now I write a lot of tests, and I continue to catch those little bugs. And I spend a lot less time dealing with old code..