How does contrast and contradiction serve as a better way of describing a

true event than merely relating what was observed?

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.

faithful servants eumaeus and philaetius:

his ragged vest then drawn aside disclosed the sign conspicuous, and the scar exposed: eager they view'd, with joy they stood amazed with tearful eyes o'er all their master gazed:

these servants know ulysses by the scar he got when he was a child - so when he showed them the mark, they immediately knew this is truly their master and they hugged him, finally starting to believe his words.