When do you need a semiautomatic weapon with a large capacity magazine that can kill a room full of people? When that room full of people (gangs or packs) are trying to rob, rape or kill you. When reloading is not an option.
I refer to Los Angeles' Watts riot, where local leaders withdrew the local law enforcement because it was "too dangerous" for the police in some areas.
I refer to New Orleans after Hurricane Katrina, where some local police left their jobs rather than protect their citizens. The police remaining (with armed military troops) followed local leaders' orders to remove the legal, lawful firearms from local gun owners, leaving them to the wolves.