Working in large codebases such as Paperback or my company's web backends has really shown me the value of writing tests. In just a couple thousand lines of code, you can figure out what broke and why. But when you get into the tens of thousands of lines across multiple programming languages, having a command you can run to confirm at least parts of your code are still working feels very very nice. I'm still not a test-driven-development person and I honestly don't know if I ever will be, but I like unit tests. We're friends.