The Norks provide much to the little weenies running China. Being a buffer between S. Korea and China is okay for starters. But what really causes the Chinese leaders to wet their pants would be nice prosperous S. Korea on their border showing China how its done. It might give Taiwan, I don't know, ideas that they can run their own country and not bow down to their future Chinese masters.
In addition, nothing makes the Chinese leaders happier than irritating Japan. Japan only makes things worse by never apologizing for WWII, but even were that never to have happened, the Chinese leaders would still being encouraging the Norks to fly missiles over Japan.
And the Norks make America look weak and indecisive because there is nothing America can do short of feeding the S. Koreans into a war just to stop the North. And now that the Norks will be able to threaten America directly, that has the Chinese leaders dancing, and distracts America from the Chinese claim to all of the S. China Sea. Without these sorts of claims, there is no reason for the Communist Party to continue to exist. They need them to generate that threatened feeling so cherished by illegitimate rulers everywhere.