Why do we use trade books to learn subject matter?

Required Texts and Materials
1. Vacca, R. T., Vacca, J. L. & Mraz, M. (2014). Content area reading Literacy and learning across the curriculum (11th ed.). New York: Allyn Bacon.
Preparation (i.e., Reading): Vacca Chapter 11: Learning with Trade Books
Assignment(s)
1. Reader Response #8
Answer the following questions using at least two citations from the Vacca text:
1. Why do we use trade books to learn subject matter?
2.What should the roles of self-selected reading and teacher read-alouds be in the content area classroom?