You will recognize this #dailysketchext if you've ever taken a college level art class before. This is standard drawing protocol (I also have drawn many piles of shoes in my life). The idea behind it is to practice the variation in line. If you notice, there aren't any shadows on this really, just thick and thin lines. This is called line nuance and drawing the skulls of horned animals is the way I have often seen this taught because there are so many ridges/ribs in the horns to work with. Line nuance used to intimidate me, but I'm pretty happy with the way this turned out.