There’s a LOT worthwhile about them, that doesn’t have to be on them. Every bit of land adjacent to water includes territorial waters, depending on what country is the internationally recognized owner of the bit of land. And beyond territorial waters, are economic exclusion zones. It’s a vast area. Once a country has a toehold on such a bit, or makes a submerged reef into one by building on it or dumping tons of rock on it, it can claim all these other territories in all directions around it. The problem that’s raised now is that no longer can the true owner, say, the Philippines, or Vietnam, expect to be recognized as such when China has enough power and money to buy the international recognition to make it “official.”
If China does this all over the South China Sea, it can completely take over the sea to the point it has power over who can transit, who defends sea lanes [or cannot] ... and all that territorial water and EEZ contains vast amounts of oil a d gas and fish and maybe extractable minerals.