you do understand how movies work?
you have a certain amount of "frames" that are shot per second.
The human eye can register up to about 25fps (frames per second)
less frames = you can see the movie switch between frames, the image stutters
more frames = smooth image; the brain records it as one smooth motion and doesn't proces individual frames no more...
so; if you shoot in 30fps you get plain and simple smooth footage...
whats the benefit of 60fps? you can slow 60fps footage down to 30fps thereby going into slowmotion but still the footage remains smooth (everything will be X2 slower)
if you'd slow down 30fps you would see the individual frames and the movie will stutter...
why not always shoot in 60fps? -> the file size will be twice as large...
hope this cleared it up for you !