Why is china building islands in the south china sea?

According to an analyst "The Chinese decide where to make islands by looking at the furthest possible range of planes, and radio stations, and then work where they need to build to make sure they have every area covered. Currently they have a neat triangle with range that extends across most of the South China Sea"

So, they are building islands to give them supremacy in the South China Sea, an important navigation passageway.