Software is not just development, there is also a maintenance perspective. If I have a bug report in a complex piece of code it's a lot easier for me to write a test that covers the use case and work from there then building, fireing up a test device, etc.

Or when refactoring a piece of code that renders UI, write some tests to verify a couple of common cases and then refactor. Run the tests and I'll know whether the output is the same.