I Know! (Being a pencil artist!).
Pencils are a mixture of graphite and clay. The more clay, the harder the pencil. Graphite is made of flat shiny particles. The softer pencils have more graphite, and I suspect, larger particles because I've noticed a different interaction between certain paper's and their tooth and the hardness of the pencil. You can layer hard on soft or soft on hard, each gives a different resulting texture. For artwork, it's important to retain the tooth of the paper to mitigate shine in dark areas. Don't press hard, use repeated gentle application and maybe blending to force the graphite into the tooth without crushing it.