What was the importance of the DMZ to the Vietnam War?

U.S. military forces were prohibited from crossing the DMZ. As a result the NVA used this to their advantage. Crossing in numbers to attack U.S. and South Vietnamese forces and their bases, then when pursued, crossing back into North Vietnam, where they knew that they would not be pursued. As a result, the NVA could re-group and resupply in relative safety. They also used this fact in bombarding bases in South Vietnam with artillery and rockets from the safety of their homeland.