QUnit is especially useful for regression testing: Whenever a bug is reported, write a test that asserts the existence of that particular bug. Then fix it and commit both. Every time you work on the code again, run the tests. If the bug comes up again - a regression - you'll spot it immediately and know how to fix it, because you know what code you just changed.
Having good unit test coverage makes safe refactoring easy and cheap. You can run the tests after each small refactoring step and always know what change broke something.
If you believe there is a bug with QUnit or would like to request a new feature, open an issue.
If you are interested in helping develop QUnit, check out our contributing guide.