I wouldn't put it simply down to just being a buffer zone, but I do think it is important for them.
China and North Korea has a shared history of politics and agendas for a long time. In the game of modern geo politics, having a well behaved NK may actually have the effect which they are looking for. A disruptive NK means that SK and the US are kept busy.
But also do not forget that China also has to protect China from NK. US plans may just destabilise the area, and I am quite sure some US Intelligence officials know this and are taking advantage of the situation.
China and North Korea has a shared history of politics and agendas for a long time. In the game of modern geo politics, having a well behaved NK may actually have the effect which they are looking for. A disruptive NK means that SK and the US are kept busy.
But also do not forget that China also has to protect China from NK. US plans may just destabilise the area, and I am quite sure some US Intelligence officials know this and are taking advantage of the situation.