Devil's advocate: I feel like TDD for iOS is a bit of a wasteland, and don't usually bother with it personally, unless there's some especially dense mission-critical business logic that I need to be 100% confident in.

If you wanna get a lot better, try writing your own testing framework. I wrote a BDD style testing library for Androod when I first started learning Java, and that helped me understand how to write good code for Android platform.

It's not simply best practice - it's necessary to maintain your code.

It is not. There are heaps of code with test which is unmaintainable and heaps of elegant code without test in sight which is easy to maintain. Cargo culting is rarely the best practice.