This is a new and interesting feature at youtube, that allows your video to be seen by only those with the URL you provided. "Unlisted" like in your phone number. It will NOT be in search or related videos and is NOT limited to the 25 users as before.
benefits / uses?
> A free video series you are now hosting on a paid host.
> You will be able to embed the video in your page now, without it being found by search on youtube.
> Will allow for comments on your blog, not on youtube.