The main purpose of using diaphragm is to transfer the lateral load to the system. This will only be used when you are having slabs but when your model doesn’t have slab then the lateral forces needs to be applied at the joint Slab possess very high resistance to lateral load as they get much higher depth in case of lateral load. But you got to transfer these load effectively to the main frame system that is shear wall and columns. Hence you assign diaphragms.
