Imagine you are a grocery store owner in some mythical place that, for the sake of this argument, we'll call "Los Angeles". Some social group gets enraged over some issue, say, for example, the results of some trial. Imagine that they start rioting, and large numbers of them come to destroy your property. Imagine that they are pulling people out of vehicles and beating them; and show a clear willingness to kill them. Now, it might be a stretch, but imagine that the police and fire departments are refusing to go into the area and stop the violence and put out the fires.
The only thing standing between your death and the destruction of your life's property is a rifle with a large capacity magazine. Not only that, but with an "assault" rifle, you can even be generous and fire "warning shots" to keep them away from your property, actually saving the lives of the people trying to kill you and destroy your property.
Now, I'm sure you'll say that this could "never happen" in America, but for the sake of the argument can't you imagine that it just might?