If you do the add host to a group element, as soon as you press the “process changes” button, and scroll back to the top of the page, I’ll bet you see the “Loader bar” telling you “Group created successfully” or something similar.
The loader is only present for 5 seconds.
The idea of the move to ajax in case you must ask is so you could process many hosts in one sitting area rather than have to recycle the load time every time you click process changes.