how does the story about the contest in the casablanca bar add to the credibility of the main plot?

To prop up his own confidence, Santiago remembers when, as a young man in Casablanca, he arm-wrestled a great “negro” who was the strongest man on the docks. The battle lasted an entire day, and finally Santiago won. For a long time after that he was known as “The Champion.” Santiago purposely recalls this memory in order to try to regain the youthful strength he needs to defeat the marlin. His memories may not literally give him strength, but he uses them to fuel his determination.

Aslan you are my hero