They provide a structured format for content syndication and now your RSS feed can use <alt> Mashup technologies to channel HTML content into them.
Link to HTML Version: http://altmobile.com/web_docs/Mashup%20Designer%20for%20RSS%20Feeds.html
Link to PDF Version: http://altmobile.com/pdf/Mashup%20Designer%20for%20RSS%20Feeds.pdf
