The history of the Rottweiler tells us that although their main job was herding large livestock, they were also often employed pulling small carts, carrying produce such as milk and butter etc. to market.
The reason why Rottweilers were commonly used as draft animals instead of horses, which could certainly pull greater loads, was several fold.
The main reason was that the dog was the more economical option under the circumstances.
Usually, draft dogs were used by small farmers with a few dairy cattle and a some chickens. Or maybe they were subsistence farmers, who used most of their crops for themselves and only sold a small amount. Usually these farmers were not big landowners. They didn't have a vast amount of products to take to market and so they couldn't afford the expense of keeping, feeding and maintaining the health of a horse. It was always cheaper to buy a good strong dog than a good horse.
A dog is not going to eat anywhere near the quantity of food that a horse would. Also, the dog wouldn’t be invading upon the cattle’s food provision. Although they are generally considered carnivores, dogs really do have some omnivore inclinations. The farmer could merely give them left overs of whatever the family was eating and a dog could always supplement it's meals by hunting vermin.
Rottweilers could serve multiple purposes. He could herd cattle from the fields, and pull the milk to market as well as being an excellent guard dog.
Rottweilers are hardy dogs and suffer few health problems, unlike horses that are sensitive beasts that could be fit one day but sick the next. (Also you don't need to shoe a dog)
Later in the 19th century, railroads almost put an end to the drover dogs in southern Germany as it became illegal to drive cattle for long distances along roads. However, Rottweilers were still used as draft dogs for several years afterwards.
The hardy donkey eventually took over the job of the draft dog. A donkey could pull more weight, was cheaper to feed than a horse, and also it could be ridden, something that you can't do to even the biggest Rottweiler.
Donkeys could also make good livestock guardians, remorselessly chasing away anything that would seem to be a predator.
The Rottweiler can still be used for herding and carting but now, rather than a line of work, it is now regarded as a fun spare-time activity for both dog and owner.