Every gun that actually shoots should be treated a weapon. This includes airsoft as well as BB guns. This means that you should always take care of your gun as though it could pose a threat to untrained users. Just as you would lock a rimfire weapon in a gun vault, locking your airsoft or BB gun in a vault keeps everyone that may come in contact with it safe.
Many people are litigation happy and a child with a BB stuck in his foot is a lawsuit for sure. Therefore, not only does a vault protect innocent people, it protects you as well.