Why are these pieces used?

I’m building the great pyramid of Giza set (2158) and it’s so much fun. I was really puzzled by some of the pieces used in the ‘under structure’. Can anyone tell me if there is a structural reason for these pieces (which get covered up) or is it just they are cheaper or something? I can’t figure out why a ‘normal’ brick wasn’t used. I’m talking about the grey sloped pieces and white rounded pieces in this picture, under the two little pyramids, for example.

The second photo I included as it just tickles me so much they bothered to put sarcophagus in there, even though you can’t see them and they get sealed up :)