Mostly around debugging and troubleshooting networking (WiFi+Zigbee network) at home, which the Zero is nice for this as it's easy to bring with me to any area in the house/yard and test stuff wherever. I used to use a laptop+radio for this, but I no longer have any laptops and the Zero does the trick nicely enough.
I also tend (try) to duplicate any keyfobs/cards I come across too, as backups, which helped me just the other day as we've lost the card we got for the municipal trash, so now I'm using the Zero to unlock them as we still haven't recovered that card.
Some months ago I used it for moving a bunch of AC+IR remotes to be connected to my Home Assistant installation by first reverse-engineering the IR protocol then building my own hardware for it with a little IR transmitter, now I can remotely control the old AC unit regardless of where I am in the house. I'm pretty sure it's a fairly standard protocol I didn't need to reverse-engineer myself, probably well documented on the internet already, but way more fun to do so yourself.