If you are actively posting to a blog, there's a pretty good chance you have posted a video from YouTube at some point.
Unfortunately, the videos of YouTube are not forever archived. Consequently, if someone attempts to view a video you posted a while back, there is a possibility that the embedded link to YouTube on your post will be unable to play the video.
For example, if you were to attempt to view my submission for the Gayest Video Ever ™ contest, you would be presented with the following:
If I may be so bold, I would like to suggest the YouTube people add one more line of verbiage to a video that fails to play: