It's a cool looking super cheap and reliable gun (ultra customizable, more accessories then a glock!) which is effective as well as fairly accurate (1-1.5moa out of any ar15 is not uncommon) and a hell of a lot of fun (low recoil helps new shooters get into the sport). You can build one for about 500 bucks right now and 30 round mags run ~8-17 bucks a piece (100 round mags run about 118-300 bucks). Ammo is cheaper then .45acp when you buy in bulk and that's in Hawaii where ammo cost more then anywhere else in the country.
you can use it to hunt, plink/target shoot, self defense, and customize/upgrade. It is cheaper then a mosin nagant (200 bucks, should be only $75-100) when you think about it after you have to buy the gun, take the time to clean it up, buy a scope (?) and pay someone to mount it (100 bucks), buy a new stock (80-200) or clean up the old one, and then you will still have a bolt action rifle with low mag round count which is shooting probably ~3moa (yes there are some shooting sub-moa but there is always an exception to the rule).
In the end nobody really needs one but don't complain if you can't defend yourself. That and its America where its not a privilege, its a right that we should be able to own one!