This is known as regression testing, and it's extremely useful. For example, you find some bug where, say, a calculation produces a nonsense result. You fix it,

This is known as regression testing, and it's extremely useful. For example, you find some bug where, say, a calculation produces a nonsense result. You fix it, then add a test for what you just fixed. 2 years from now, someone refactors the code and reintroduces the bug because their refactor didn't take into account all of the possible input cases. Your test now catches it. Our regression tests have literally found hundreds of bugs over the years and saved us a lot of time and embarrassment.