When you ask about “the communists,” I assume that you are asking about the communist countries, China and the Soviet Union, that helped North Vietnam in this war. These countries helped North Vietnam because North Vietnam was also a communist country and was therefore their ally in the Cold War.
In the Cold War, the United States and its allies were in competition with the Soviet Union and its allies. The Soviets wanted to spread communism while the US wanted to prevent communism from spreading. Each side used various tactics to try to achieve its goals. One of the tactics that the Soviet Union and China used was to give military help to communist insurgencies that were trying to overthrow non-communist governments. This is essentially what was happening in the Vietnam War. North Vietnam was supporting a communist insurgency in the South that wanted to overthrow the South’s semi-democratic government and make the South communist as well. By helping North Vietnam, China and the Soviet Union felt that they would be helping communism to spread. This is why “the communists” wanted to help North Vietnam in this war.