People can, and do defend themselves against home invaders. Not everyone that breaks into your home has a gun either, sometimes they are unarmed or have a knife.
Better to be armed than risk having someone hurt you or your family.
Having a firearm doesn't help or fix every situation but it puts the odds in your favor.

And the most important thing to remember in all this is the deterrent factor. Criminals would rather not hassle breaking into homes where the likelyhood of the victims are armed is high.

Just like mass shooters would rather find a gun free zone where they can murder at will, unchallenged, at least until the guys with guns show up( police).