In the tutorial, it specifically describes Groups as, "…documents [that] always have a â€œdisclosure triangleâ€
It provides another visual indicator that what you are looking at is a folder intended to hold content, as opposed to a document which [i]can/i] hold content.
I emulates the behaviour that users are accustomed to in the Finder, where even empty folders have disclosure triangles.
When dragging, to place a document inside another, you have to drop “on”. When dropping into a folder, you can insert it by slotting it in underneath as a child - this is only possible when there is a disclosure triangle. Whilst the result between dropping “on” or “under and to the right of” an empty folder is exactly the same, it means that all folders behave consistently during a drop operation regardless of whether they have children or not, again emphasising that they are “containers”.
So, this design choice was based on consistency rather than function.
Hope the UI police won’t shoot me for this one.