Here's a simple test to do with the demo. Try tracking that motion-blurred cube on the left with your eyes - it's all blurry. Now, try tracking the one on the right - it will be sharp, but "jumpy". Now, try tracking the one on the right with your 120Hz monitor - the motion should be MUCH smoother - and that's where the need for high frame rates comes in. The higher the frame rate, the better the object will look when you're tracking it with your eyes.
Ideally, if we only account for objects travelling across the screen in 2 seconds, and a screen is ~2000 pixels wide, and we want to have an error of <=1 pixel, then theoretically, we need (2000px)/(2sec) = 1000 Hz frame rate for high motion realism. Unfortunately, I don't see it coming anytime soon...