I don't know if these are uncommon but I have a few cool usecases besides the regular 1:1 folder syncing, maybe someone else finds them useful.

Also you should know that the way I have all of this setup is that I have a container that hosts a bunch of SMB Network drives and a syncthing container that stores all of the fodlers on that drive. Having them also easily accessible through smb is great when I just wanna quickly copy something or back the folders up.

So here are some of my maybe unorthodoz usecases :

• Music - As a fan of offline music, I have it setup that music I acquire gets synced onto the server and re-encoded as opus through a script into a second folder which then through syncthing gets sent out my mobile devices. There I rather have smaller files than lossless quality. Said script also sorts the music into folders based on artist and album metadata.
• I also sync my Newpipe Subscriptions between phones (unfortunately by manually exporting my settings and re-importing them)

I also used to have a setup that would sync Minecraft Bedrock and Stardew Valley saves between devices (where Windows and Android saves are compatible) but Android 11 introduced a stupid restriction that prevents synching from accessing the the saves are located on Android.