The response to the previous occasion when women and children were taken captive for sacrifice was an impassioned and successful counter attack. This time, it is a full retreat. The new information that we have here is that in the retreat they took “all the inhabitants with them, both in towns and villages.” Once again, the details that Mormon presents allows us to understand that these are the unusual cases. We are therefore to understand that the typical mode of retreat left the inhabitants of towns an villages in place while armies fought around them. This is, in fact, the typical mode of warfare known from that region of the world.
In this case, something has changed. What has changed is the sacrifice of women and children. Since the Nephites can no longer protect them by driving the Lamanites from the land, they must protect them by removing them from the possibility of capture. Mormon gives us this information because it is a change in the way Nephites fought wars. It is a logical tactic in response to the sacrifice of the women and the children.