When you diagram a sentence, you not only show the relationships among all its different parts, but you also show an awareness of the function of each part. If you can do that, you'll gradually develop the ability to write better sentences, which will lead to better paragraphs, and, ultimately, to better papers.
The following exercises are designed to progressively lead you through all the various sentence parts that exist.
Excerpted from the Student's Resource Book, pp. 588-97.