Perhaps I simply want to know where I'm headed before I spend 20 minutes of my time. Not all videos do a good job explaining the plot in the first 30 seconds. With this I can get a TL;DR instead of skipping 1/3 through the video to see what it is actually about.

Also note, youtube is directing producers to make entertainment-only content and that stuff has very little to summarize. It's viral because it's entertaining, you wouldn't TL;DR it.

I think the "Steve Jobs' 2005 Stanford Commencement Address" is a perfect example of a link someone sends me that I want to preview before I schedule my time watching it.