Answer: To entertain
Explanation: The primary purpose of the crab that played with the sea is to provide entertainment to the readers as the author does this by the imagination of the creation done by the eldest magician.
The excerpt is given as thus: "Before the high and far-off times, o my best beloved, came the time of the very beginnings; and that was in the days when the eldest magician was getting things ready...one by one he took all the beasts and birds and fishes and told them what to play at"