Take your pick:
(1) Back up the Norks?
(2) Make sure the Norks don't think that in case of disaster they can withdraw into the safety of China?
(3) To show off their new tank designs ?
(4) To remind Kim that he can make no move without permission from China.
Probably to back up the Norks. If not, why put all that hardware there if there is a risk of losing it in a nuclear response from the US?