What is the MAIN reason why the author included the information about desertification?

Probably to try and portray and image in the readers mind to allow them to try and relate/understand the situation of the environment which they were in. Basically to give them a better understanding, many authors do things like this.

While winters are rainy, summers are dry. regions with this climate are usually in the eastern part of the continents. they experience hot, humid summers and colder winters. precipitations have a regular distribution throughout the year, but hurricanes are common.