Frame-rate switching allows you to have a smoother video playback experience.
It's very noticable on a 4K tv and when playing 23.976, 24, 24, 29.94hz video content that always play at 60hz without frame-rate switching.
Then it causes micro video stutters, camera zooming from left to right not smooth and audio becoming out of sync after a couple of minutes.
Frame-rate switching eliminates these problems and the screen first switches to the same frame-rate that the video is encoded in.
