"How does a weapon help you protect property when the law does not permit a citizen to use deadly force in the protection of property?"

Deterrence. There's no guarantee, obviously, but most people are likely to reconsider targeting a given place if there's an immediate threat of being shot. Doesn't really matter whether the person hoping to deter vandalism would be in the clear for doing such a thing, it just matters that the vandal(s) believe the person might do such a thing.