It's not so much about playing/slowing/speeding up an animation or video. It's about moving forward and backward through an "experience," as much as I dislike the overuse of that word. I'd suggest it's a natural evolution of the scroll behavior.
I remember as a kid asking how many possible speeds there are between 60 mph and 0 mph. Infinite right? So how does the car get from 60 to stopped when mom hits the brakes?