Hardship During the Great Depression
Seabiscuit began to race during a time in American history when people had lost everything they owned in the stock market crash. Thus, Seabiscuit became a horse that represented the people of that time, he was undersized, seemingly outmatched and had his soul and spirit crushed by life. With the help of Howard, Tom, and Red, he learned what it meant to be a horse again, and because of it the American people found hope in themselves once more.
Restoration
We see Howard lose his son, Red lose his family and have to fight his way through life, Tom emerge as a loner who everyone believes is a crackpot, and Seabiscuit have his identity beaten out of him. The film takes us on a journey of restoration of the heart as each character must have dense portions of stone chipped away in order to reveal the life the still resides within each of them. Each character helps others to do so.
David vs. Goliath
Seabiscuit is smaller than all of the horses he races against, especially War Admiral who is a prime physical specimen. Thus, their race represents a David and Goliath battle, and with Seabiscuit winning the battle the story becomes complete, just like the original version.