I'm watching engineers re-implement error handling code I wrote line by line over the weeks after they "optimized" my error handling into a oneliner. And as the old bugs crop up when API error responses are weirdly formed, they find themselves adding the edge case code I had added previously.

If I had been granted time to add unit tests, those would just function as a source of truth: "sometimes the API returns this kinda weird error, so we handle it. Sometimes this one, so we handle it." Unit tests are nice for that, all things this given program (the UI in this case) needs to worry about from the various things it talks to (it could talk to a couple different APIs who all have different quirks).

I wasn't granted time because the API quirks are considered bugs that are being fixed... one day... hence why the oneliner "refactor" was allowed, but regardless, it has been my go to object lesson in why I finally find unit tests useful.