The best use of subpages is to organize hierarchical content into a hierarchy or outline. We can already do this simply by using the HTML heading tags (h1, h2, h3, etc.) as is done in most items posted here at WPMU.org and in this post, but subpages add the additional abilities to place them on the menu, in a sidebar menu, and provide navigation between them.
What are subpages and why would I use them? – WPMU.org