Why manually test just a couple conditions after each code edit when you can let a test suite check hundreds of conditions every edit!?

This is another tool for detecting errors earlier, where you’re more quickly able to identify and fix them. It’s a bit more work up front, but can save you a ton of time later.

The key when writing your tests is to verify the behavior of your functions, not how they’re implemented. In other words, your test shouldn’t know anything about what happens inside the function; it just knows the inputs/environment and checks the outputs.