In my experience at least, the two primary benefits (and sometimes, the only benefits) of re-encoding are 1.- reduced file size and 2.- increased device compatibility.
The file size is relevant because you can fit more stuff for essentially the same quality: reencoding a FLAC album to ~160k Opus uses up only 1/5th to 1/4th of the space, ~196k Opus is 1/4th to 1/3rd of the space, so it can be a pretty good gain on aggregate. A movie in 4K is worth nearly 6 movies in 1080p and nearly 9 movies in 720p, and for ~95% of extant content in the world rebasing down from 4K to at least 1080p presents no practical loss.
The compatibility is usually only relevant when you want to have that content be easily accessible in eg.: a remote media server, a streaming system, or one of those gool ol’reliable MP3 thumbsticks. In those cases, you’d be reencoding audio from FLAC to MP3 to increase device compatibility (and getting some decent space savings too). If your Jellyfin server’s connection is over wifi or you’ll access the data outside of your local network, re-encoding to lower sizes means transmission requires less bandwidth, as well as other savings (incl.: energy consumption in aggregate).
In my experience at least, the two primary benefits (and sometimes, the only benefits) of re-encoding are 1.- reduced file size and 2.- increased device compatibility.
The file size is relevant because you can fit more stuff for essentially the same quality: reencoding a FLAC album to ~160k Opus uses up only 1/5th to 1/4th of the space, ~196k Opus is 1/4th to 1/3rd of the space, so it can be a pretty good gain on aggregate. A movie in 4K is worth nearly 6 movies in 1080p and nearly 9 movies in 720p, and for ~95% of extant content in the world rebasing down from 4K to at least 1080p presents no practical loss.
The compatibility is usually only relevant when you want to have that content be easily accessible in eg.: a remote media server, a streaming system, or one of those gool ol’reliable MP3 thumbsticks. In those cases, you’d be reencoding audio from FLAC to MP3 to increase device compatibility (and getting some decent space savings too). If your Jellyfin server’s connection is over wifi or you’ll access the data outside of your local network, re-encoding to lower sizes means transmission requires less bandwidth, as well as other savings (incl.: energy consumption in aggregate).