There is a test for that. Try building a tennis game, just in code with no gui, and take note of how much you would either write tests for things like the scor

There is a test for that. Try building a tennis game, just in code with no gui, and take note of how much you would either write tests for things like the score, set points etc., or how much time you spend debugging to justify if your code works. Unit tests are a great documentation tool as well, both for developers but also for the business needs of the software — the value.