You're right, I didn't 'get it' until I worked at a place that was all TDD and I was surrounded by people who took it seriously. It wasn't about 100% coverage or any forced mechanics. It was simply a group of people who were all on the exact same page working on the same exact methodology. Almost a 'cult' of programmers.
> i haven't worked out how to test-drive that
That's exactly it. I don't always see tests as being necessary for greenfield code. You don't have to test drive it. But, you should write tests. Once you figure it out and have things in a place where you're comfortable, write a test so that when you go back and make a change later... you know that the code will break tests and you can be confident of 'change over time'.
> i haven't worked out how to test-drive that
That's exactly it. I don't always see tests as being necessary for greenfield code. You don't have to test drive it. But, you should write tests. Once you figure it out and have things in a place where you're comfortable, write a test so that when you go back and make a change later... you know that the code will break tests and you can be confident of 'change over time'.