Just for the fact that the chickens would make the feeder filthy if it is left on the ground is the main reason to hang it. I hang mine just about their shoulder height. If you hang it too low they will just waste a lot more feed.
I have all of mine elevated, acomplishes the same thing. It does eat up floor space, but how many hang out under the hanging ones. The only problem I have found is that in summer, it gives bugs a place to hide. Move the feeder, chickens clean up bugs.