Dynamic display block slideshows are mainly used to showcase featured content at a prominent place on the frontpage of the site in an attractive way. A featured content slideshow can contain e.g.: New articles, Most popular content, New products, Portfolios, Videos etc.
A lot of news sites like CNN, Yahoo, BBC and other sites use advanced slidehows to display content. Advanced slideshows are often made with flash. These slideshows can be very sophisticated with all kinds of effects. But content used in flash is not detected by search engines and your site will not be Search Engine Optimized. If you are CNN or Yahoo this is not that important (people find them anyhow), but for most sites SEO is one of the most important features of the site. With the Dynamic display block slideshow you can now use advanced slideshows while the content is detected by search engine.