The building to convert reef to island is one of the first steps to claim the entire Southeast-Asia sea by China. It will then have the self-declared ownership right to block all sea/air traffics through this region and bully other neighboring countries. Besides having a huge oil/gas reserve and fishes, in term of economic impact, this sea is as important as or more than Panama Canal or Suez Canal. A country with the right to block or own it will harm the rest of the world. Currently, the allies have been formed with a group of countries Japan, Philippine, Viet Nam, America and others trying to stop China but they have not found any solution nor success yet.
Here is the map that China wants to claim: https://en.wikipedia.org/wiki/Territorial_disputes_in_the_So...