I worked as a draftsman long ago. One of the techniques draftsmen learn is to apply a systematic ‘hierarchy of lineweights’ to enhance and clarify their drawings. Every draftsman learns to do this. It is important because it adds clarity and a kind of richness to drawings. You can also see it at work in comic books and skilled illustration of all kinds. There are probably several different systems for deciding on lineweights. Here is mine. I am showing it in a very simple way, but like a lot of seemingly simple things, it adds more than you would think.