It allows china to press economic claims in the surrounding waters. Such as exploitation of resources, and interfering in commercial shipping.

Overall it's a chess move. It allows China to exert pressure on the region. Similar to Iran and the strait of Hormuz.

Useless islands are good for expanding Exclusive Economic Zone.

https://en.wikipedia.org/wiki/Exclusive_economic_zone

The minerals, fish and shipping of the area makes up most of the other countries economies. With control of this area china could exert pressure directly onto these other countries.

$5 trillion a year in cargo passes through.