The point of it is: they were going to do a skeuomorphic animation, and they put the effort in to do it right. It's a level of polish that I really appreciate, even though it isn't really necessary for anything.
It is one of the best, if not the best, way of letting users know what their action will lead to, and of letting them explore the range of available actions.
By behaving similarly to a well-know interface, it simplifies discovery, lowers friction and reduces frustration.
So when smartphones and touch interfaces were new to most people, behaviours such as this one were major, although nearly invisible, product features.
Now? Yes, it mostly isn’t necessary to anything, but it was nice.