To begin, the Willenbrinks explain that "A structural sketch will help you observe and understand the underlying structure of the subject,"(Willenbrink 16) and also that, "A structural sketch helps you see how a subject is constructed"(Willenbrink 17). With this, the Willenbrinks are essentially saying that structural sketches is a good technique for beginners to see what makes up the object that they intend to draw, similar to how shapes give an idea of how something is structured, and that it is a good way to warm up before doing a sketch or drawing in another style (Willenbrink 16).
The reference picture that I will be using is this one:

So, the first step that is instructed is to,"Sketch the outer forms of the mug and the most relevant structural guidelines, such as those that indicate the placement of the rim. the bottom of the cup and handle"(Willenbrink 17).
Here is a picture of the structural guidelines of the cup
And here is what it turned out to be
This exercise was also a bit difficult for me to use. There were a lot of lines so sometimes I would get stuck trying to figure out how to fit the object within the lines, but the lines did give me an idea of where what should go.
What do you think about this technique? Do you think it'd be useful to you or would it actually make it more difficult for you to draw?
Willenbrink, Mark, and Mary Willenbrink. Drawing for the Absolute Beginner: A
Clear & Easy Guid to Successful Drawing. Cincinnati, Ohio, North Light
Books, 2006.