Mainly started off with using Plex since shows that I wanted to watch were simply not available on any service available to me. Now it gets used when the service where the content would be available craps itself.
Otherwise I use it to host a nightscout service for my diabetes management since the CGM manufacturer dropped support for an old phone I kept around for no other reason than to use the CGM since the new phone I had was never supported to begin with. Granted now the manufacturer came out with a standalone receiver, but why bother when I can just use my regular phone now?