http://googlesystem.blogspot.com/2010/06/youtubes-interactive-transcripts.html

"YouTube [has] added a cool feature for videos with closed captions: you can now click on the "transcript" button to expand the entire listing. If you click on a line, YouTube will show the excerpt from the video corresponding to the text."

I'm not sure is this new and fresh

Example video provided (i.e. found) by the source:

http://www.youtube.com/watch?v=IY3U2GXhz44

It's available for days now. I'm still searching for any interest for that, as I bet your screen isn't big enough to see both the video and the transcript together without scrolling.

Jérôme even created a post on this forum about that, three days ago.
http://blogoscoped.com/forum/171472.html

What is the use case for this?

"If you text it, they'll read it?" ;-))

Tom:

Someone sent the tip 5 days ago, but I couldn't post it at that time. Here's a use case: search inside a video using your browser's find feature (Ctrl+F). Another use case: music lyrics / karaoke.

Thanks for pointing to the specific thread (the duplicate finder doesn't generated a warning either) and for the background info.