Yes, "sugar coat", saying it was a farm dog is true, trying to spin it into Lassie herding ducks is ridiculous. They were catch dogs. There were too rough on stock to be allowed to run the farm and were typical tied to a tree or building. Research the English "bandog". I understand the Pitt's fighting heritage offends your sensibilities. I don't condone dog fighting. I also accept the true history for what it is. I'm not anti-pitt, I am pro reality.
Rotties tails were indeed docked to prevent injury. Mostly from bashing their own tails against building / fencing. One look at a dog cart and you clearly see it would be near impossible to get a dogs tail caught in a wheel. Of course anything could happen. The biggest reason the practice caught on was the lack of Med. Care for dogs in the 1800's. For a dog or livestock, an infected wound was a death sentence. This is also why ear cropping was done. Pigs are nasty animals a licks bite and off comes a dogs ear. So drop eared dogs got their ears cropped. A practiced associated with fighting, but originated with farmers a shepherds.
The small farms used dogs to pull the small carts on the farm. Rotties were used more as cattle drovers, carting was a side benefit. Farmers realized that with the tail docked the Rotties trotting endurance improved. That was due to the shift in the balance point allow the spine. Without the tail that balance point moves forward. In Nordic breed that run long distance the tail set it held curled over the spine which also accomplishes the same thing. Rotties are extremely territorial , so they filled a role as farm security too.