It's to allow you to capture and output video through analog means with the appropriate dongle. The only other requirement is installing the video capture driver (WDM driver), which is available on the CD that comes with most video cards. Of course, if you have firewire available, you may opt to use that instead.
but there is video in, and video out, i know what video out is now, using computer on tv etc, but what exatly is video in for, what can it be used for?