Both effectively 
Primarily ordering subflow instances in a group.
I can now remove hundreds of duplicate node groups in a dashboard flow because I can pass suitable data to instances - thats grreat but nodes appear in a random order and the dashboard side control cant re-order them because being dynamic they do not exist. Yet. But if I give a priority to each instance for the group to build up the right ordering then that would be amazing.
Now if you also wanted to be able to give a 'sub'priority to each dashboard node in a subflow that would be even better because if for a particular control you wanted a Button and a Slice - you can either group and order them, or by making the offset bigger than the instance priority set, group each type of node. Presumably you would apply it as an offset (or give a calculation so you could be more flexible with the layout)
