Reliable Code

When you have a solid test suite, you can refactor your code or add new features without fear. If you break something, the tests will tell you immediately.

Tests are the best documentation. They show exactly how your functions are intended to be used and what they should return for given inputs.

Writing tests forces you to write better code. If a function is hard to test, it's usually because it's doing too much. Testing leads to clean, modular architecture.