What is the main purpose of the round steel cylinder surrounded by artificial islands

Most of the artificial islands are reefs or shallow cofferdams blowing sand from the island (such as China's islands in the South China Sea) its advantages: conducive to the consolidation of national defense, to ensure the safety of the sea border, conducive to the sea fishing fishermen to rest to avoid natural disasters such as typhoons, to the passing ships to replenish the materials used, conducive to the convenience of installing navigation to lead the lighthouse (to ensure the safety of the sailing ship) people in the sea there is a survival fulcrum. Disadvantage: far from the mainland center of the city, the supply of materials, fresh water resources are relatively scarce, to live on the island travel inconvenience.