One example of this could be the two node types "sling:Folder" and "sling:OrderedFolder". The later one will keep the order (and care about the order) of the nodes inside it. This can be visualised when you e.g drag the different pages of your side in a special way and they end up with a special order in your menu (eg. if you get all the pages under a certain node and add them to the menu list). Whilst if you would have all these pages in a normal folder or under a unstructured node, there is no way that you could know which order you would retrieve them by by just iterating over the sub-nodes.
Hope that helps
Johan
