Freefilesync gives you a (good!) gui to observe and then resolve differences between file trees. This is useful if the keep-latest strategy all of those 'blind' sync tools doesn't (always) work for your use case. Freefilesync has let me catch accidental deletes countless times for instance. I use Syncthing and rclone as well, but for some use cases I need 'eyes'.
Overall, FreeFileSync is more like Unison [1] than like Syncthing or rsync. Though, I haven't really used rsync, so I can't make a detailed comparison.
Syncthing is automated/daemon, so the syncing happens in the background automatically. FreeFileSync's default behavior requires manual start/stop of sync jobs, though it does have an optional real-time sync feature.
The main advantage of Syncthing is that it supports multiple hosts/locations for the same backup, and that syncing is P2P between the hosts.
