I've been writing software for decades now, and I find that it's quite the opposite. Other than using an SCM, nothing has been as much of a game changer for me

I've been writing software for decades now, and I find that it's quite the opposite. Other than using an SCM, nothing has been as much of a game changer for me as automated test suites. I'm able to tackle bigger and larger problems with more confidence. I can tell when my new code has caused an error in some dusty corner. I more easily think through edge cases and sad paths. I can show someone my tests and they can understand what the module is supposed to do. For the record, I don't TDD/BDD much anymore. I do use the lessons I learned while I was to write more testable code.