The short answer is that online communities are engaging ... Visitors enjoy the instant communication with people from around the world who share their interests along with the instant gratification of finding exactly what they're looking for the way they're looking for it ... something that just wasn't possible with the ole static non-interactive sites. From a business perspective, visitors spend a long time interacting with your company and your brand (which is HUGE), fresh new content is produced constantly, and the content is user-driven so your website produces content without all the man hours of writing it yourself.