You have to take a look at a map to really understand Taiwan's importance.

Taiwan isn't about military proximity. It's about access shipping access. Try open up a map. Despite China having a vast coastline, they do not have access to the open seas. Every one of their shipping lanes requires passage through another nation's waters.

If a heavy conflict were to erupt, China's supply chains would be cut off via naval blockade. It's a huge risk to China, and one they've attempted to ameliorate via the Belt and Road Initiative.

That changes if they acquire Taiwan. Taiwan's importance is not of offensive, but defensive primacy.