One example that we explicitly brought up here when talking about this was hotfixes for existing issues in prod. If you find some urgent bug that needs to be fixed ASAP, you write a test that isolates the bug, make your fix, and verify that the test passes. You probably don't want to wait 45 minutes for the rest of the test suite to pass before deploying your critical fix to prod (though there's really another issue there, as well, which is that your test suite takes 45 minutes.