I have two cases where I use one-way sync in Syncthing.
One case is syncing photos made on my mobile devices. The folder I use for this is receive-only on my server (well, a Raspberry PI in a cupboard, but I call it a “server” for the sake of cool), send-receive on phones, and send-only on a tablet that has tiny storage. There’s a cron job (actually, a
systemd timer) on the server that moves the photos from this folder to photo collection folder that is then synced to PCs and laptops. Having the folder RO on the server makes sure the photos don’t get deleted on mobile devices as soon as they are moved off to collection (and by the time I remove them from mobile to free up space they’re safely in the collection already), and having it SO on the tablet makes sure the storage space on the tablet is not cluttered up by the photos made on other devices.
Another one of my usecases could actually do without one-way sync, but I still have it just to make sure. The abovementioned server has a backup system set up on it (which includes
rsnapshot, a RAID of attached disks, and some other things), and since it is also a 24/7 Syncthing instance, all the shared folders are backed up, too. Hovewer, I have a couple directories on my home PC and my primary laptop that are not shared with other machines. To have those backed up too, I shared each one of them with the server, so that the server always has an actual copy that it can back up. These are SO on the machines. This could be a two-way sync, of course, since the files are not meant to be modified on the server anyway, but I have set them to one-way sync just to be double-sure.