That same one, or a different one, gets used as my 'house' device on WiFi only, so that my primary phone's battery gets used less and is therefore less likely to need charging if and when I need to leave the house.
I have one as a semi-hardware cryptocurrency wallet that can go online if necessary, but primarily is offline.
I also used to use one GPS spoofing in Pokémon Go - I actually found the intellectual challenge of getting it working far more satisfying than the actual act of playing Pogo 'remotely'. But I'm a couple of years 'sober' now and have no desire to regress.
I've also used a phone or two as webcams / security cameras. Permanently plugged in, but with the app ACCA (requires root) which limits charging to within certain battery percentages to minimise the chances of spicy pillows.