With your dog a helpless victim, it's no laughing matter. If I'm a crook and your dog is out, your fence protects ME, not your possessions or your dog. If he's tied up and I keep out of reach, he's useless. He'll bark, but outside dogs bark so much, they're usually ignored. I can't hurt the dog until he can hurt me, and nothing you own is worth my arm.
Deterrence is effective protection.
Protection and aggression are not the same. Protection is defensive, reactive, often passive, and threatens or injures no one. Aggression is active, harmful and offensive, threatens all and benefits none. Yard dogs often develop far more aggression than protectivity because everyone who passes by or enters has already violated the territory that dog has marked dozens of times a day for years. First, property owners have implied social contracts with others in the community.