Ernest Hemingway's The Old Man and the Sea
takes place in and around Cuba. The love of baseball began to grow in Cuba during the late 1800s, and by the time of the action in The Old Man and the Sea
, baseball had become a national sport and pastime, much like, say, hockey in Canada or soccer (football) in Brazil.
So an old Cuban fisherman who talks about baseball is realistic in Hemingway's setting. Santiago's continual references to Joe DiMaggio, though, go much further than just establishing a realistic setting. To Santiago, Joe DiMaggio — who was and still is considered the greatest baseball player in history — represents what a man should be. Santiago idolizes DiMaggio in part because he (DiMaggio) suffered through the pain of a bone spur to make a great comeback.
This idea of struggling and persevering in order to ultimately redeem one's individual existence through one's life's work is central to the conflict of The Old Man and the Sea. As Santiago struggles with the marlin, he equates his struggle with the pain of DiMaggio's bone spur and tries to live up to DiMaggio's example by not giving up on the marlin.