The old man's significance seems to lie more in what his presence reveals about the father and the son and their differing reactions. The old man is described in terms that deliberately highlight his plight and his suffering, including a powerful simile that describes how broken he is as a human:
He looked like a pile of rags fallen off a cart.
Yet note the two differing reactions that the father and the son have to him. In spite of the old man's suffering, the father has had his humanity and ability to empathise eroded by this new, dystopian world. The first thing he thinks when he sees this old man is that he could be a decoy sent by a group of humans to trap people like themselves. When they begin to talk to him, the father says, "If this is an ambush, he goes first." This clearly indicates the way in which the father's sense of basic humanity has all but disappeared.
His son, on the other hand, still possesses that goodness and simple decency. It is he who recognises that the old man is afraid, and it is he who manages to persuade his father to give the old man something to eat. In this section, as in elsewhere, it seems as if the son plays an important role in keeping the father's lingering sense of humanity alive and not letting him become the monster that all other humans seem to have turned into. Thus the old man's importance lies in what he reveals about the relationship between the father and the son.