Taiwan remains to be a de facto separate political entity, refusing to be reunited with the PRC under the leadership of the Chinese Communist Party (CCP).
So, China not allows Taiwan to sit in any international body where statehood is required, thus making Taiwan rather isolated internationally.
The tension in the Taiwan Strait would affect not only both sides, but also the whole Asia-Pacific region and beyond.
China has to deal with not only its counterpart in Taiwan, but also the US. Taiwan issue is still one of the major bones of contention in Sino-American relations.
The US has been blocking Chinese attempt to use force or threats to use force against Taiwan. US using Taiwan to restrain China’s rapid expansion of power.