What is China's objective in militarizing the South China Sea?

I was reading a presentation from Capt James Fanell (retired director of intelligence and information operations at U.S. Pacific Fleet) on China's Navy:
He made this statement which caught my attention.
Transcript: The Rise of China’s Navy: A Discussion with Capt. James Fanell
China built between – late 2011 and through 2015 seven artificial islands in the South China Sea. And three of those islands are the same size and dimensions, in terms of geography, as Pearl Harbor. One of them is the same dimension as the beltway that goes around Washington, D.C.
Given:
- Chinese president Xi Jinping in 2015 promised not to Militarize these Islands but they have done exactly that.
- That they have been harassing US navy ships operating in the region even coming close to raming a US destroyer.
- That they have alienated many of their neighbors who claim the waters these islands inhabit.
- That they are risking confrontation with Japan and the US. Japan having the largest most sophisticated Airforce in the region. The US having the 1st and 2nd largest air forces (USAF, USN) in the world. And China has been actively threatening both.
- China now has more hulls than the US Navy.
- China is out producing the US navy in tonnage of ships. and will for the foreseeable future.
- China has superior anti ship missiles than the US Navy (Range, Speed, Power )
China says the sea is part of their historic territory, a claim refuted by the Permanent Court of Arbitration in The Hague. Western Intelligence experts asserted in 2011, China is after the natural resources in the South China Sea ( oil, natural gas, fish ).
Question:
Given we now understand that we badly underestimated the scale of China's militarization in the South China Sea and Navy, detailed above which seems like overkill to get natural resources; Is there a different strategic objective which they could be after by controlling that swath of ocean?