It’s a very good question, and the answer at this point is that if you use smartthings you are able to create much more complex rules than you can with the other systems you described. And you can also add Zigbee and Z wave devices if you were interested in that.
But if the combination that you have does everything you want it to, there’s no reason to add a smartthings hub.
See the following FAQ for more details. (The topic title is a clickable link)