It sets the framerate to the same as the movie - if you play a movie at the wrong framerate it's going to judder as frames are skipped or repeated. If you have the GUI at 60fps and don't have AFR on then you'll play a 23.976fps or a 24fps movie at 60fps causing a mismatch of frame rates and frame judder.
It's the same in the Shield Pro. If match frame rate isn't turned on in Plex it will stutter on many 4k remuxes. Especially DV.