I use my own time to work on tooling I miss on my workstations/own servers [0]. I also have a knack for finding weird bugs in otherwise widely popular and stable software [1,2]. I spend some time on select subreddits (WireGuard for instance) and on IRC to help out when I can.
I do it mostly because it's fun, and I learn some new stuff. Not much is applicable on my day-to-day job though.