Expansion of power and intimidation of their neighbors. Why else would they bother. It's either for oil/gas or a military base. And in the case of the Spratly Islands, it's both.
I'm all for China trying to become an economic power, but I draw the line at them becoming a true military superpower. They already have nuclear weapons. We must contain them as much as we can. It's in our best interest until they can become a true democracy (if ever). Honestly, after Islamic terrorism, they are our biggest threat in the region.