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. Letter carriers, paperboys, delivery people, law enforcement, emergency medical personnel, meter readers and others are allowed near and at times on your property without your specific permission. Second, even if the intruder is a criminal, few places allow you or your dog to cause physical injury to prevent property loss.