A few honest questions

first I'll start by stating that I've never owned a lophophora. why do so many people seem to use just coarsely crushed lava rock as a substrate? drainage? aeration? also, why do so few people fill the substrate to the height of the plant that mimics nature? does it promote the formation of pups? maybe if I'm lucky, I'll get one or some seeds. I'd like to be well informed on the off chance that I'm able to get one.