As to your question, there are any number of reasons, from nefarious to benevolent...often a mix (never let a good crisis go to waste). The public reason is usually security. But, I think China wants more.
China has an imperialistic strategy. Xi has a dream of "One World, One Dream." He seeks absolute preeminence in the world. He believes it is China's birthright. Manifest Destiny, if you will. By 2050, after the PRC turns 100, it will be the world's dominant superpower.
Unless, that is, the United States acts to oppose it beforehand...and by oppose it, I mean economically, politically, and militarily, but hopefully in a successful Cold War II. Trump started this Cold War by reversing actions taken by globalists Clinton, Bush and Obama. It remains to be seen if the Bidens will take their China money and thumb their noses at China, or in return for that money, undo what Trump has done, and hasten Chinese dominance.