The island is a very bizarre and unusual inclusion in the book; up until that point in the novel, everything seemed pretty explainable. Even though improbable, Pi manages to surive with a tiger on the lifeboat; he does this through implementing his knowledge of animals, and the importance of the alpha male. This makes sense, and he explains it pretty well. However, when we get to the island, there is no reasonable explanation. An island of carnivorous reed-like plants that floats in the middle of the ocean and also houses lemur-like animals that climb trees? Very bizarre indeed. So, why did Martel include it? What was the symbolic significance.
Martel's entire story is about the importance of stories, and how believing in stories makes our lives better. He parallels this with religion, or faith in God. He says that life without God, or without purpose or a religious story to explain why we are here, is a "dry, yeastless factuality" that is very difficult to cope with. Religion and faith make our lives easier, and the stories behind them explain our purpose and direction. But, in order to believe the stories behind some of the religions that exists, it requires a pretty huge leap of faith. You have to believe in things that are bizarre, miraculous, and oftentimes unexplainable. That is just like the island.
Up to this point in the story, we can go along without doubting things too much. But, to explain the island, we just have to take a leap of faith and believe that something like that actually exists. Of the island, Martel said,
" the island represents something that is just beyond what can be reasonably believed...the island represents the leap of faith, that if you choose to believe it, you must abandon your excessive use of reason."
So, the island was put in for the purpose of making the reader take a leap of faith, and believe in the story, which is a major theme of the novel. I hope that helped; good luck!