I have unit-tested my code for many years.
While building a GIS-system, we really cared about our product quality. Our users’ needs demanded the app to work properly. I had all critical and/or complex parts of code 100% test-covered, with multiple paths and corner cases. It was such a pleasure to find a bug, fix it, write a couple of tests for this surprise scenario, and be sure it won’t break again. Ah, good times.