Why do they cut off the tails of some breeds of dogs?

Most breeds with docked tails are because the breed historically was developed to do a job where their tail was at significant risk of injury. For example, the Rottweiler was used to herd cattle. A cow stepping on a dog's tail can literally rip it off. In some cases, it can take part of the rest of their spine along with it. A coworker's undocked Australian Cattle Dog had precisely this happen last year. She lost all function in her rear end. They ended up euthanizing her. Jack Russels were used to hunt rodents, often underground and in other tight spaces where a long thin tail could potentially be prone to injury. In addition, handlers dig the dog out once it finds its prey, and a long tail could be at risk of being seriously injured by the shovel. The docked tail also gives the handler a sturdy "handle" to get hold of when removing them from a hole they can't turn around in.

Wow...I didn't realized so little people knew this..

The main reason a tail is docked is to prevent injury to the tail. Spaniels and other hunting dogs get their tails docked to prevent it from getting caught in shrubbery when they're running through trees and bushes. Herding dogs get it removed to prevent it from being injured by the animals they herd. Fighting dogs or guard dogs get it docked to prevent an offender from grabbing on to it. Other working dogs, specifically dogs that were used to pull carts, get it done so that it doesn't get in the way from them and the cart.

In other words, it was done to protect the dog. Other reasons people had to dock a dog's tail was to prevent rabies, strengthen the back, increase speed, cleanliness (feces stuck to a bushy tail) and to prevent health problems. The health problems thing is about how some dogs were bred to have little importance in the tail and keeping it would result in a weak tail and future health concerns.

Nowadays, the purpose is almost always for appearance. Showing a dog has nothing to do with docking. Dogs that normally get their tails docked are still allowed to enter a competition if their tail is unaltered. Most countries, including the UK and some parts of Canada, ban the practice. It's rare to see a dog with a docked tail in the UK. Unlike here in America.

For many breeds of dog, it was originally - and often still is - done for working purposes. For certain herding breeds, getting tails caught while on the job was (and sometimes still is) a very common occurrence. Docking the tail before they get it caught prevents them from being able to get it caught and having to get it amputated. This kind of docking was and still is often seen in some hunting breeds, as well. It was, basically, to prevent injuries to the dog during herding, ratting, baiting, and similar jobs. Interestingly... it was actually believed at one point to prevent rabies.

For show dogs, it is done mostly for aesthetic purposes. Many standards call for docked tails and therefore, tails are docked while the puppies are young enough for the procedure to be performed. This is more common in certain countries (the US) than it is in others where it may even be illegal (the UK). Of course, there's controversy over doing it for nothing more than aesthetic and therefore, in many countries it's actually banned unless under special circumstances.

The other reason for docking is health reasons. This is more or less amputation rather than docking. For example, if a dog suffers a really bad spider bite on his tail, the tail may need to be amputated. Or if a hunting dog catches his tail in a brush and suffers a real bad injury, the tail may need to be amputated.

Historically, it was done to prevent injuries in certain working breeds, if you have ever tried to get burrs out of an undocked Spaniel's tail or tried to lift an undocked terrier by theirs (yes, sometimes you have to drag/lift a working terrier out by the tail to save their life, this is much easier if there is a sensibly docked tail and not a long, hairy thing)... it was/is very functional for certain lines of work.

Most dogs today, however, are not working dogs in the traditional sense. In those dogs it is done as a tradition and for a few it is required for conformation showing (fewer now that docking is banned in most of Europe). There is statistical evidence that traditionally docked breeds have more tail injuries when their tails are left on - most breeders feel that a virtually painless procedure when the pups are 3-5 days old before nerves and blood vessels have fully formed is preferable to the later potential of the dog requiring full anesthesia to amputate a fully developed tail due to recurring injury (tail injuries are often recurring and very painful).

Depends on the purpose the dog was bred for.

Ratters have shorter tails to make it easier for them to get in and out of holes/burrows and if needed for the hunter/owner to pull them out.

No "fighting" breeds have docked tails.

Some hunting breeds have their long whip tails docked to prevent damage while out in the fields.

Dobes have theirs docked because the breed originated as a guard dog. One less thing the attacker could grab onto.

BTW not all dogs with nub tails are docked. There are several breeds who naturally carry for the bobtail gene. Pembroke Corgis, Catahoulas, Boston Terriers etc

Also there are breeds who are docked who have longer docks - the poodle for instance.

Originally it was for working purposes, so the tail didn't get in the way e.g trampled on my cattle or grabbed by a fox. These days it is banned in many countries, however in America they still do it to keep in with the show standard.

I however am glad it is banned, I love rotties with their tails!

Working purposes. It's especially important for herding dogs so their tails aren't injured while herding livestock and cattle.

Its pretty much just because they breeds are "supposed" to not have tails. There really isn't a good reason to cut off a dog's tail, since the tail provides an animal with better balance, so without a tail, that dog is at a disadvantage until they learn to survive without a tail.

some times the tail will grow long and get stuck or when that dogwags its tail is may hurt if it hits you

Some breeds chase their tails & actually bite them off anyway over time, so that's a good reason.

These midget dogs with their flat faces, stump legs & deformed brains have nothing but problems throughout their lives. It's the same as breeding neurotic kids with harelips & club feet.

It's the worst form of cruelty to animals, to breed deformities for your own visual pleasure.