This exercise is a first step to learning how to tackle the unexpectedly overwhelming concept that is texture and detail. We'll start out by learning to study a reference image and to work past our all-too-human instinct to throw away critical visual information.
Then we'll explore applying what we've learned from our reference images as a tool - not bound by how it is presented and arranged in the reference, but in full control, using it to convey information exactly as is needed to get an idea across to our audience.
Remember that you should be doing this exercise with a fineliner/felt tip pen. If you're having trouble filling in the dark areas, a black brush pen can also be very useful.