If you remember that the family left Switzerland to begin a new life in another place, the ship contained what they needed to start that new life. However, when chased by pirates they crash landed on the shore, and the family needed to make critical decisions. To erase all trace of their living place, the family needed to move off the beach, taking with them all the supplies so that the pirates would not find them. If they were to live and survive, they needed to escape from the shore, find a place where they could not be seen from a passing ship or pirates, take all the provisions which had survived the landing, and build a home in the woods where they would be invisible from the water. They also needed a defense system in case the pirates ever came to the island looking for them, and that they could only really build away from the beach.