I have to say that I don't quite understand where a lot of the discussions on this thread is going, so I'm going to push this a different direction. Why is south china sea important and how can China utilize it?
The first question is quite easy. Most of the energy exports from the middle East and Africa passes through the region on their way to East Asia. Similar, all of the trade activities pass through the region on their way from Europe. Right now, USN or any of the countries around there like Australia, Malaysia, Thailand and Indonesia can easily choke off that sea lane in the even of a major dispute. There is not much China can do about that. That's a huge security threat for China striving for the super power status. There is always talks of energy exploration in South China Sea, but I don't see that as important as China's trade dependencies.
The second question is more interesting. This is a good article explaining the strategic importance
. As long as China has a major military presence in the water with the carriers and submarines, it will be hard for any foreign power to display the military bases they have already set up. Just as importantly, they can attempt to neutralize the vast submarine threat by building an underwater. For example, they can fully built out (if they haven't already) an vast undersea hydrophone network to detect not just surface fleet but also submarines. That along with all the ASW helicopters from heliport in the region and surface warships would allow them to more easily track the quietest submarines. Something that would be hard to do with their diesel submarine fleet.
More importantly, the Spratley bases have already finished construction
. They can house 72 fighter jets along with 4 large aircraft. In the even of a war, that could mean 30 J-20s, 36 J-16s, 6 J-16Ds, 4 Y-20 variants. It could even be temporarily used to refuel H-20s. I estimate Spratley bases to be about 1000 km from Singapore or the straits of Malacca. Any Chinese military presence in the region will need to be able to provide legitimate threat to that particular area. 2 large carriers + 1 LHD would allow them to carry an air wing of probably 70 J-35s, 30 J-15 variants, 10 KJ-600, UAVs and maybe 50 helos of different variety. If we combine the air force and the naval presence, they would have numerical advantage in 5th generation aircraft, AWACs, EW aircraft and heavy 4th generation aircraft against individual countries in the area and probably even if they were to face 2 USN groups along with its allies. PLAN carrier groups alone would be quite the force. The addition of J-20s, J-16s and Y-20U or Y-20 AWACs would allow them to maintain numbers in the region. In normal scenarios, I'd say that 1 J-35 on a carrier would give them more air time than 1 J-20 flying from 1000 km away. However, the permanent air bases allow them to quickly rotate J-20s from other regiment into the regions. If they have 300 to 400 J-20s overall, then maybe 250 of them would be available for operation at any time. They could essentially rotate the available J-20 units through Spratleys 2 brigades at a time, so that the pilots and the aircraft can have some downtime afterward for relaxation and maintenance. Of course, they could also rotate carrier groups, but that would take longer time. With enough Y-20Us from Spratleys and Hainan, J-20/J-16s with drop tanks could loiter for some time even if they have to fly 900 km into the theater.
The other thing is that Spratley base also give them the possibility of bombing Australian naval base at Darwin, Cairns and maybe even perth at light load. Since Australia is likely to help USN in any war scenario, PLAAF has to be prepared to bomb naval bases where Australian submarines or surface combatants operate from. With refueling along the way, they can probably carry sufficient payload to effectively attack those bases. Without refueling, they'd be more limited. Realistically, the only way they could potentially take out Australia from battlefield is to drop a lot of bombs in those regions. You don't really want to send a carrier group there while advanced submarines are actively operating in the region. They don't really have to worry about long range land based SAMs around Australian naval bases. I don't know how much Australia has invested in tracking VLO aircraft. There is a good chance they have not done much investment due to lack of such threat. As such, they would only need to fly around any surface combatants (like Hobart class) with advance air radar and SAM.