Life changes. People change. Ideals change as society grows and matures. What was good for mankind years ago may not continue to be good for people of today. To ensure we protect our freedoms we need to evaluate what ideals we still believe in.
When we know the purpose of the different text structures we can write in the best format to attract our intended audiences. Understanding the "how" and "why" of other author's writing - helps us to do the same with our writing.