It describes why the author writes about the events and their causes.
Explanation:
The author explains the reason why he began writing about Yellow Fever. The book by John H. Powell, which he read some six years ago was an eye-opener to him.
The book made details about the Philadelphia epidemic more real to him and so, he decided to begin writing about the plague. He, and many other writers referenced Powell in their works.
I believe they are common because people believe that is where we could be headed. Will we run society into the ground, or will we prosper? Will the society be a "Divergent" or "Giver" society or will it exist as it does today? Since we have no way of knowing, we pretend we do.
