The main reason is the mouse. In this example Silverlight application Amazon Search Visualization
the user is able to search the Amazon catalog by creating relationships using the mouse to drag and drop (see this link for more Silverlight examples). A properly designed interface that uses the mouse allows the users to be more productive.
Why DotNetNuke and Silverlight?
Silverlight is an application that runs in the users web browser. It has it's own operating system and one would be tempted to create solutions completely using Silverlight. For example, lets assume you wanted to create an application that allowed users to schedule meeting resources. The users would load the Silverlight application and use drag and drop to indicate the items they needed and to design the room set-up.
However, you would quickly realize that different users would need access to different options. How do you store these options? The logical place is your DotNetNuke website because it has an extensive user and roles management system that also handles your normal web pages.
How do you communicate with your DotNetNuke site? Using web services (see IWeb for code) or WCF is easy to implement.
Even with the dramatic changes in user expectations that Silverlight is expected to bring, DotNetNuke remains an integral part of your total solution.