Hello
I would take it that they are trying to get you to learn about body flow. Many beginners get bogged down in trying to put too many details into their work, by that I mean they portray the body almost as a stactic frame. You practice drawing the quick line without looking forces you to define your body shape as a flowing line of the motion formed by pose.
I’m certain you’ve seen artwork where someone was swinging a bat or running, and it looked alright but you got no feeling that the person was actually running; but that they just struck a pose as if they were.
Understanding to replresent how the body is flowing will help your art avoid that stiff feel.
They may also be using this as a means of ‘loosen’ up your drawing style, get you to think free drawing rather than say technical illustration.
Also drawing the quick line can teach you not only about flow and motion, but train you how to see things like center of mass in a pose, or distribution of weight. Forcing you to draw the quick line making you percieve not the body as a whole, but how that body’s mass is postioned for the pose.
Mr. D