Those routed designs in the stone appear to serve multiple purposes, the surface routing looks like open face molds for casting metal objects, some look like they are designed to mount either decorations or interlocking connectors and some appear to be conduits to carry a liquid through the stone from one place to another.
One can surmise a possible explanation to be a holistic approach to construction using every inch of the constructed object to serve a purpose, thereby using limited space and resources more efficiently. The hole routed through the stone had what appeared to be a gouge running the length of the hole which is indicative of lubrication being pumped into a well fitted bearing. These are only my assumptions looking at the videos but maybe it will strike an idea in someones mind and they will figure this thing out.