Someone please explain how I can use unit testing or test driven development w/ codeigniter

[eluser]tomdelonge[/eluser]
I understand the main idea of writing a test and then writing code to make the test succeed. I've even setup Eric Barnes simpletest for codeigniter library.

I think the trouble I'm having stems from the simplicity of my website. It's rather simple, more like an ecommerce store with some minor differences. So I understand I could write an auth library and test logging in and such, but how do I test other aspects? What else am I supposed to test?

I'm trying to learn how to use unit testing - is it possible that my website is so simple that it doesn't need it?