I've mainly use Flipper Zero to duplicate my digital apartment keys (iButton then later RFID fobs). It's so easy to duplicate a physical apartment key, but making backups of the digital equivalents is annoyingly tedious. Plus, apartment managers treat them as scarce commodities and refuse to give backups.

With Flipper Zero I now have backup keys in my backpack, on my dog's leash, in my running belt, and with close friends. It's great.