@Hamed Got it. Thank you!
So the Get Array Value at Index Behaviour is creating a locally stored variable/pointer of whatever its lookup (get) from the array returns, for the use of local Box Containers, Value Behaviours and any other localised behaviours wanting to use it. I think.
For anyone coming along in the future, this seems to be what's happening, in pseudo code (please correct me if I'm wrong, @Hamed), wherein * indicates a pointer.
*myLookUplocalStorage = myArray[atIndex]
myBoxContainer = *myLookUPlocalStorage
myArray[atIndex] += 1
// myArray[atIndex] is now 1 bigger , but neither localStorage nor myBoxContainer know anything about this update
myBoxContainer stores a pointer to the localised lookup (get), so whatever changes at that address is instantly reflected in myBoxContainer