Syncthing just... syncs things. Say you have a folder that you want to automatically get synced between devices, syncthing is exactly for this.
If you want something like Google Drive, you can run Nextcloud, which is like a self hosted Google Drive, but more powerful. You upload files, which get saved to the server, not just synced between devices. Then you can also sync them, sync calendars, news (RSS feeds), edit documents in it (assuming you install the correct extension), and a lot more things.
Yep, it's centralized. However, it offers more functionality than just syncing stuff. If you only want to sync files, syncthing is the simpler, more lightweight solution :)