When the author relates a particular event, he encourages the reader to follow along. When this happens, the reader is meant to focus on the story and understand the events that transpired. However, the reader might get distracted, or he might fail to identify the specific aspects that the author wants to highlight.
By using contrast and contradictions, the author encourages the reader to focus on a new idea that is significantly different to the one already presented. By doing so, the author forces the reader to pause, rethink and reevaluate the information presented, thus increasing his understanding of the subject.
