What is the strategic purpose of China's sea-forts?

To probably cheese off its neighbors.

They have several objectives: 1) China has polluted their country and waterways just like the US did in the 50's, 60's and 70's which has killed a significant amount of fish / food source. They are expanding the islands to create a larger fishing area. Just last week, Japan protested but did not respond militarily as China sent 230 fishing boats and seven armed coast guard ships into Japanese territorial waters (within 22 kilometers of land) around the Senkaku islands on August 5th. Japan issued an official protest which China dismissed because China does not recognize Japanese claims to Senkaku. Now it is up to Japan to act, or not act, to stop the Chinese from physically taking possession of the Senkakus and the surrounding waters. 2) It will expand their defense and allow for projecting force, but that will cause a bigger issue with the US and allies.