Speaking with my experience from the intranet side, DNN allows us to create specialized applications that are unique to a page. I have built many custom modules for the intranet. A module is just a web application, and can exist on it's own "page". It can also co-exist with other core modules on the same page.

For instance, I created a budget and payroll tracking module that exist on the "Budget" page. But also on the same page are other mods like an FAQ, links, and contacts.

The DNN also allows the ability for your custom modules to tap into the framework and take advantage of built in features such as role based security, ajax-like controls and data access.
BOL