The reason North Korea exists is because it makes a nice buffer state between China and South Korea.

North Korea could always count on Chinese support because the last thing China wanted was a US satellite state on its doorstep.

An "independent" North Korea means the United States doesn't need to worry about a military incident involving Chinese troops.

The problem now is that this.  North Korea has become somewhat erratic and unpredictable.  And so both China and the United States have something to worry about.