Read the excerpt from "The Crab That Played with the Sea." And the Rat of the Moon stopped biting the line; and the Fisherman let his line down till it touched the Sea, and he pulled the whole deep Sea along, past the Island of Bintang, past Singapore, past Malacca, past Selangor, till the canoe whirled into the mouth of the Perak River again. ‘Kun?' said the Fisherman of the Moon. ‘Payah kun,' said the Eldest Magician. ‘See now that you pull the Sea twice a day and twice a night for ever, so that the Malazy fishermen may be saved paddling. But be careful not to do it too hard, or I shall make a magic on you as I did to Pau Amma.' Then they all went up the Perak River and went to bed, Best Beloved. Based on the details in the excerpt, what is its primary purpose?