Under the condition that the stress mentioned by your teacher is not ideal, if the force can't only act on the center of the structure, then the external force will produce a moment for the whole structure, so many members including the zero bar will produce internal forces, and even many members without bending will bend. At this time, the zero bar will play a role in resisting external force deviation and ensuring the relative stability of the structure. It is equivalent to zero bar blocking external force inside the structure.
Of course, the zero pole is not omnipotent, it does not resist the side. Therefore, the actual structure is not a plane structure, but a spatial structure is more stable. Members bear external forces to ensure the stability of the structure in all directions.
The above is my opinion, and everyone can criticize and absorb it. Hope to adopt!
You can ask any questions!