system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is the "One-China" policy so important, to the US, to China?

I can't seem to wrap my head around why this is such a big issue.  From everything I have read it seems to be a policy around political posturing with no real benefit to anyone.
gpt
The Communist Party considers Taiwan part of China.  
The Communist Party also considers several places namely Xinjiang Province, Inner Mongolia, Tibet and Hong Kong as part of China and actually runs those places.  
The people in those places don't like the government in Beijing very much, they are all quite different from the Mainland Chinese people, but they still pay taxes and follow the laws.  
If China just lets Taiwan go, all of those places would start getting ideas . They might all try to break away.   
  
If that much territory revolts and breaks away, some provinces might think that maybe the whole Communist Party is weak, so they might consider looking elsewhere for leadership. Conveniently there is the previous government of China still living in Taiwan.    
  
So basically, Taiwan being recognized and leaving would likely spell the beginning of the end of Communist China, something they will absolutely go to war over.