From what I understand - you don't test for all unknowns as that is humanly impossible. You test for realistic "known" only. This largely prevents regression bu

From what I understand - you don't test for all unknowns as that is humanly impossible. You test for realistic "known" only. This largely prevents regression bugs as new fixes or features are added. When a bug reveals itself and is fixed, it becomes a known case that you can decide to create a test for (if necessary) and prevent its recurrence in the future. Personally, I feel it improves the code quality and readability for others as the tests also help them better understand the functionality of the code within a project.