Why does anyone need a Rolex watch or diamond ring? Why does anyone need a fir coat, speed boat, ultralight plane or motorcycle? Why do you need a car that can go faster than 80 mph or can quarter mile in less than 18 seconds? Why does anyone need 140 channels on their TV? It is not a case of what people need. This is the USA. If you want something and can afford it who is the government to say they can not have it because they do not "need" it? No one needs an automatic rifle but if you want one you should be able to get one.
Owning an automatic rifle requires a special tax stamp, which is expensive and difficult to get. Few citizens legally own automatic rifles. But to your point...have you ever fired one? They are pretty cool. I don't own one, but I fired them in the Marine Corps.
I used to shoot competition full auto, my take is full auto is one of the most useless weapons there is.
Unless you are going into combat,,,PFFFT,,,just a waste of ammo.
Even combat, my choice would be semi auto or bolt repeater.