Like most of the world, China is fed up with North Korea, which yesterday (May 29), in defiance of international pressure, conducted its third missile test in three weeks. The unpleasant neighbor is steadily progressing toward its goal: the ability to hit much of the world, including the US mainland, with nuclear-tipped intercontinental ballistic missiles.
But, for Beijing, North Korea’s saber-rattling does serve one useful purpose: It distracts attention from the contested South China Sea. Last year, the world fretted over China’s territorial aggression in that resource-rich waterway, with critics warning it could become virtually a “Chinese lake.” Beijing claims nearly the entire sea, based on what an international tribunal ruled last July to be bogus reasoning, both legally and historically.
Now, thanks largely to North Korea, the issue has faded into the background—just as China might have hoped.

https://qz.com/990701/north-korea-is-helping-china-in-the-south-china-sea-whether-it-knows-it-or-not/