@renardroi Construction lines are supposed to help you understand volumes and how to flex them and turn them. The small circle is a cutaway / cross section through the skull sphere to make it narrower/straighter and is a simplification of the shape of a real skull.
@thor I'm very aware of the purpose of 'construction lines' (I use them often), maybe you missed the part where I said this was homework. As I discussed with another person, and mentioned in my post, I don't take issue with construction lines in general, just this particular method which has so many lines as to make it confusing, as well as being drawing shapes by protrusion rather than subdivision (the latter is easier to keep proportionate). Thx for the slightly condescending explanation tho.
@renardroi You can draw without constructing the volumes first, but you're more likely to make mistakes without noticing it, and if you're not already thinking in 3D when you draw, constructing your volumes can help you have that a-ha moment. All the old Disney animators used these techniques.