Initially, when doing puppy training, we keep the puppy on a leash to teach it not to run around causing havoc, and to stay by our side. When doing leash training with a puppy, the primary objective is to train the puppy to walk at our side, to follow when we turn and to stop and start when we do. This part of leash training a puppy is often accompanied by commands as the goal is ultimately to be able to walk with our dog without a leash. In reality however, this objective is seldom achieved, not because the dog is incapable, but because we tend to not spend the time and effort on leash training a dog to get to this level.
When we however talk about leash training a dog to become a guide dog for the blind, the leash takes on a new meaning. In this context, it could well be said that the dog is taking the owner for a walk. Not in the sense that the dog pulls the owner everywhere it want to go, but the leash is used by the dog to indicate to the owner where to go. Examples of this is in order to avoid obstacles in such a way that the owner can safely negotiate them, and in a traffic situation where the dog is trained to intelligently decide if a command given by the owner is safe to obey. In this context, special training leashes as well as harnesses both during leash training and the actual execution of the work.
A third aspect of leash training comes to the fore when dogs are trained for attack work. As the natural aggression of the dog is stimulated with K9 training, the leash is here used to forcibly restrain the dog from actually attacking until the command is given. Special training leashes and collars are used for this in order to prevent mishaps due to the dog's strength and agility.
Whatever the exact circumstances and purpose for leash training dogs, it is absolutely essential that leash training is in fact done. Whether or not we take it as far as progressing to off leash training (where you teach your dog to walk with you without a leash) is immaterial. Other than the guide dog situation described above, you really don't want to take your dog for a walk without having done dog leash training. The pulling in all directions, dog and owner getting tangled up in the leash and the dog charging any other person or dog in the vicinity does not make for a pleasant experience and will damage your bond with man's best friend.
Other articles on this site that might be of interest include: Leash Training