Why do we use leashes? Mainly, it is because they are required by law in most places and are used for safety. Often the leash is also used for control. We like to think of the leash as like a seat belt, used for safety and not for control. You hope you won’t need it but it is there to protect you and your dog if you do need it. If we teach our canine companions how to walk on a loose leash and to respond to our voice, rather than a tug on the leash, the leash becomes just a safety device. At first, some people have difficulty seeing themselves and their dog getting from where they are now, to having a loose leash and working more as a team with their dog. With some patience and training change does happen. A wonderful illustration of this concept is described in the graphic below by Lilly Chin. If we can teach a whale to pee in a cup, a sea otter to go to his crate, a rhino to smile for teeth-brushing; without force or control, then we should also be able to teach a dog to walk politely on a loose leash without force or punishment.
By the way, thinking about leashes, if you like to read and would like to better understand canine behavior, The Other End of the Leash by Patricia McConnell is great book that is very informative and an easy read.