Maybe the 3D Builder is something we can use when they give away all the HoloLens units to the Insiders....
Edit: I do see the folder and it is hidden and locked up. Since the basic apps like Spartan are in a Systemapps folder, I will assume this folder is for other, possibly purchased apps.
I think it i something not intended for mour view or consumption. It will probably be gone in the RTM. But Saltgrass's guess is on the mark. It is immense inside, with sub,sub folders. Here is a very small portion: