My father was a bird-hunter. We always had at least one one Brittany Spaniel living with us. There was one puppy we got whose tail wasn’t docked, so he had it done. Many today are born with a naturally docked tail.
Why is this done? Their tails, if long, have long hair hanging down them, somewhat like a horse’s mane. When the dog goes to fetch a shot bird, it is often bounding through brambles that catch the hair on the tail. It can hurt the dog, sometimes even trapping it in the weeds.