Why does the author include the story of the Spanish Galleon in the novel, Misty of Chincoteague?

The Spanish gallleon is the boat that the first horses arrived on way back in the history of the island. This boat crashed and sank along with the crew and only the horses ended up surviving the wreck. The author's purpose in including this backstory on the horses is to show the type of adversity that the ponies must have had to endure and to set the scene for the unbridled and stubborn attitude of the wild horses on the island.