I assume they are going to pour concrete over that system. Seems it would be more cost effective to form that with a boards which would allow closer rebar spacing, and reuse of the form boards. The cost of the clay filler would be eliminated. The system also puts the rebar closer to the top of the concrete slab rather the bottom where it would provide much more strength to a suspended slab.

What is the purpose of the clay filler block? What am I missing here?