China help negotiate the terms of full US disengagement, paving ways for eventual communist victory.
https://en.wikipedia.org/wiki/Richard_Nixon's_1972_visit_to_China
Thirdly, Kissinger and Nixon wanted to get help in resolving the Vietnam War. By dealing with Russia and with China we hoped to put pressure on Hanoi to negotiate seriously. At a maximum, we tried to get Russia and China to slow down the provision of aid to North Vietnam somewhat. More realistically and at a minimum, we sought to persuade Russia and China to encourage Hanoi to make a deal with the United States and give Hanoi a sense of isolation because their two, big patrons were dealing with us. Indeed, by their willingness to engage in summit meetings with us, with Nixon going to China in February 1972, and to Moscow in May 1972, the Russians and Chinese were beginning to place a higher priority on their bilateral relations with us than on their dealings with their friends in Hanoi.