Originally Posted by Lutherf

The most common reason for wanting (needing) a semi-automatic weapon is to defend one's self from an attacker armed with a semi-automatic weapon and/or multiple assailants. Since the early 1900s the semi-automatic handgun, and later the semi-automatic rifle, have been the most commonly produced firearms. As such, they are the type of weapon one would most likely encounter in an attack. Being armed with a single shot pistol in an engagement against 3 attackers puts the defender at a serious disadvantage.

This disadvantage was recognized well before the invention of the semi-automatic firearm. It was not at all uncommon for an infnatryman using a musket to be supported by a loader and two or more backup weapons. The infantryman would fire a round, hand the musket off, take a loaded one and fire again as the first weapon was reloaded. In later years this lead to the development of the cartridge which improved loading times compared to component loads.

I would also note that firearms are not required to commit a mass casualty attack. In 2014, in Kunming, China, 8 assailants stabbed to death 31 bystanders and injured another 140. The attackers, by the way, were stopped by a responder with either a fully automatic or semi-automatic firearm. THAT is the purpose of modern firearms and that is why self defense minded people want and need them.