Why does anyone need an AR-15, AK-47 or any other semi-automatic weapon? Simple. If there is a government, you need it.
Of course the issue is not "need." The issue is a "right" as in "the right to keep and bear arms." Totalitarians (i.e. all leftists) hate that "right." Just like Obama, Hitler, in 1935, surrounded himself with little children when he announced the gun ban was for their safety. The Hitler Youth, like Obama's civil defense force, prefer you to not have a weapon.