Probably because you don’t want your IP address to be associated with some of stuff on there.

I do something similar and use syncthing to sync a folders on my phone to my server, I then use beets to manage my library (helps pull correct metadata) and jellyfin is just pointed to that folder. There’s a fork of the syncthing app on fdroid: https://f-droid.org/packages/com.github.catfriend1.syncthingandroid/