Lesson 6 and 7 use a lot of complex construction techniques where it's valuable to be able to draw lightly, so as to be able to distinguish the object from a forest of construction lines. It's also expected by that point in the course that you'll have learnt line confidence and ingrained the ghosting method from using a fineliner. For example, this one I drew in my submission would have been very difficult to clarify if everything had been the same weight and darkness because of the insane amount of subdivision on the box.