Marshall Kirkpatrick of the SplashCast Blog has written an insight into using tagging to collect and share videos.
In this instance he is discussing tagging as the process of finding a video and bookmarking/saving it along with keyword/s. So for example you may find a cool Jim Carrey video and tag it as "Jim Carrey", "Actors" and "Comedy".
You can now use the video in your own library, or as part of a web TV channel you share with other people. If you or your friends wanted to see a library of Jim Carrey videos then that video would be added into the library since you tagged it as "Jim Carrey".
As you, your friends, and possibly other people in a wider community go around tagging videos you can end up with a much larger video library/channel that contains only videos that have been tagged with something that interests you.
This is all just a more personalized and social way to consume video.