When you play 24 frames/sec on 60 frames per sec TV, some frames need to be displayed twice in the row and some 3 times to keep everything in sync. Because of that when for example panning the scene, the picture will visibly speed up and slow down, depending on if you see 3 or 2 frames. You could partially fix that, by interpolating, or creating extra frames in between, positioning moving object half way in between real frames to smooth it out.
Playing 24 frames at 120 frames display, you only need to display 5 times each frame since 120/24=5 exactly. So you don't need to create any frames in between to smooth it out, the motion is as smooth as the original movie.
And again this is how I understand it, but I've been wrong before.