well, you can see what people are searching for and not searching for and the volume. For example, one of the sites i run is for an adult entertainer micah moore, her site came out in late 2007. if you search for her name on google trends you see
Google Trends: micah moore
Also, it lets you compare multiple sites or queries to see if there is a correlation. just for fun, you can check valentines day and lingerie
Google Trends: valentine, lingerie
it also seems to track traffic as well, check when i put my site into the websites search function
Google Trends for Websites: abadss.com