Simple answer is that Chinese military deployment in SCS is for national security to defend attacks coming from Pacific Ocean, securing maritime trade route, and national integrity.
South China Sea ans East China Sea are both main entrances to China via maritime route.
Brief of SCS conflict history
Since China was found, 9 dash line (used to be 11 dash line) has been claimed by China according to a per-defined 9 dash line recognition between the US and KMT. Since the US supports KMT rather than CCP during civil war, and the US did not expect that CCP would finally win and rule mainland China.
The 9 dash line would be the great interests of the US if KMT won and rule mainland China. However, the dream was awaken when CCP won the civil war, 9 dash line thus against the interests of the US, because CCP is not sided with the US.
*note. for 9 dash line illustration which is not 9 dashes (see above) is not correct. This is the reason why it was called 9 dash line.
The US has China condemnation strategy of Island Chains to prevent China from entering maritime.
First Island Chain starts from Japan, linking via Ryuku Islands, Taiwan, Philippines, Malaysia, and ends in Vietnam. This seals up Miyako Straits and South China Seas, maritime doors to China.
Second Island Chain starts from Japan, linked via Northern Mariana Islands, Guam, Palau, ends in Malaysia
SCS tension, Why Now?
Prior to 2000, China has insufficient navy power to protect its claim in SCS. There was no base, no alliance country to provide navy base for supplies. This made SCS a property with guard.
From SCS confrontation between China and the US, China found its disadvantages. Without a decent navy, it is impossible to protect SCS interests. It spent 2 decades to develop its navy force and air force, building artificial islands in SCS as bases for providing supports to navy deployed in SCS, thus give China permanent supplies for defense force in SCS.
SCS tension was not recently raised. It has been a issue since China found. It becomes hot when China has sufficient power to defense SCS militarily.
This is the main reason the US tried to bad mouthing China’s sovereignty in SCS, supporting Taiwanese party which promotes Taiwan independence, associate Philippines to claim SCS against China (because the US has nothing to do with SCS, it needs a proxy to act on behalf of the US), insists that Diaoyu Island belong to Japan. All these mentioned are for one single purpose, to prevent China Navy from entering Pacific Ocean that the US will loose control on China.
THE IMPORTANCE OF SCS TO CHINA
SCS is a major maritime route for cargo transportation between China to the rest of the world. Without being able to secure the safety of the route, it is national security for China, if the US sealed the route militarily.
In terms of self defense, SCS serves as early warning point if military threat coming from Pacific Ocean. With bases in SCS, China can detect any treats from Pacific Ocean early enough to prepare for defense.
As the rapid development of Chinese Navy force, the First Island China has been broken. China has been exercising Miyako Straits navigation by Air Force strategic bombers and fighters. While Chinese attacking submarines with ICBM capable to launch 10 warheads ICBM undersea, now going in and out via SCS into Pacific Ocean.
This is a threat to the US mainland, as Chinese attacking Submarines 094, 096 are capable to bypass the US detection and depart from Sanya base through SCS into Pacific Ocean without trace. In other word, the US mainland is facing possible retaliation attack by nukes launched anywhere in Pacific Ocean and land on any city of the US.
In terms of economics, SCS is an important transportation route to ship goods between China and Far East, Middle East, Africa, and Europe. China has to be able to secure the route by its Navy for the success of Belt Road Initiative (BRI), which the US is trying to undermine for its own interests.
Also, SCS contains many energy resources, natural gas, and crude oil. This is the key reason causing hot dispute in SCS sovereignty.
National integrity is the top priority among Chinese policies over weighing any economic and financial benefits.
Thus, China has been actively negotiating with involving countries and willing to share the resources among countries, because the benefits of SCS to China is not purely energy resources, while to other nearby countries, resources is the sole reason to claim SCS.
Above information should be able unlock many questions about China-US conflicts and the recent US actions in SE Asia.