Skipping collaborative apps, these days even a single user needs to sync his/her devices. The device landscape is getting predominantly mobile, so an average user has multiple powerful devices with unreliable internet connections.
Popular solutions suck for one simple reason: common sense fails us in distributed systems. Proper sync needs some math like a GPS device needs some relativity formulas. Yes, that is just a box that says your x and y. No, it can’t work by common sense alone; it needs math.