If I'm a crook and your dog is out, your fence protects ME, not your possessions or dog. I can safely shoot, stab, spear, poison, snare, strangle or dart it through the fence. If it's tied and I keep out of its reach, it's useless. It'll bark, but outside dogs bark so much they're usually ignored. I can't hurt it until it 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 and 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. Letter carriers, paper boys, delivery people, law enforcement, emergency medical personnel, meter readers and others are allowed near and at times on your property without your specific permission.