A completely useless button from YouTube.

I have observed on the left corner of a video from YouTube (played in HTML 5) a little button, just near the play button. I wanted to see what it does, so I moved the mouse over it, and a bar with a tortoise on left and a rabbit on right appeared. So, I slid left and right, and the video slowed down and accelerated. A really useless button, in my opinion. Who wants to do something like that? It only makes the video...uninteresting.

You can try it while viewing this video: http://www.youtube.com/watch?v=GYFEIw8OLQs. (make sure you have HTML 5 activated; verify or activate here: http://www.youtube.com/html5)