Spratlys: Why it's important

aside from the petroleum/natural gas deposits under the sea floor, there is another reason why China would like to control it.

Belmont club has this map:

For supertankers with deep drafts, the lane is actually smaller, because of the coral reefs and islands;


and of course they are trying to take over all lands that any Chinese government has owned over the last 2000 years...

they claim the Philippines, but actually it was part of the Caliphate, not china, so we are probably okay:but Viet Nam was once a Chinese colony, as was much of Korea, so you can see the problem.

and the dirty little secret is that China did not control the West Philiipine sea during the last 400 years: It was controlled by Chinese pirates, who bribed the Chinese officials to look the other way

pdf here has more.
and it's not just the WestPhilippine sea where there is a narrow sealane that makes ships vulnerable to pirates: 

and the BBC notes that it has increase in the last several years.

Another big danger point is off Malaysia, which is why Singapore has conscription and it's own army/Navy.