The face is a very expressive part of the body, both human and dog. Have you ever noticed that your dog has reacted adversely to black faced dogs?
The face, the tail, and body posture are how dogs mostly read each other when on approach. Unfortunately, black faced dogs, or those wearing a muzzle or nose harness, are at a disadvantage. Subtle facial movements such as eyebrows and eyes are not distinguished so well. Not being able to read the whole face may make other dogs wary or even reactive.
Also, think about the dogs who have their tails docked by humans. This is also a disadvantage because the tail is very expressive in showing a dog’s state and its emotion.