The straight construction lines help a lot when you are drawing the symmetrical object model whose two sides are alike; such as straight lines help you draw the ovals properly.
”Is a top larger than the bottom, or not?”
“Are the sides straight and parallel, or are they slanted?”
“Do the top and bottom lines slant or are they horizontal?”
“Do I see a true circle, or an oval?”
Be sure you really know the shape of the model before you draw a line on your paper. Look carefully, be sure of what you see, then draw it. These are the fundamental steps in making a good drawing.
The straight lines help you draw the shapes that you seem more easily will stop block them in likely.
The final touch darkened some of the contrast lines for interesting accents.