Gadrev

Microsoft Silverlight is a cross-browser, cross-platform plug-in for delivering the next generation of media experiences and rich interactive applications (RIAs) that incorporate video, animation, interactivity, and stunning user interfaces. The fast installation with a small, on-demand, easy-to-install plug-in that is less than 2 megabyte (MB) and works with all leading browsers. The consistent experiences between Macintosh and Windows-based PC. Vector-based graphics, media, text, animation, and overlays and improved performance for AJAX-enabled Web sites.
You can also find the top ten list of reasons from Tim Sneath.