As a rule, we must carry the dog on a leash in public places, and there are also very few dogs well trained to walk with their owner or stay still in a place without attending to other stimuli, such as another dog, a cat or a ball. So it is important that learn to walk on a leash from very young, because they are going to use it a lot.
Ideally, all dogs could walk freely, but especially in the cities the rules are strict, and not wearing it on a leash can lead to a fine. That is why a dog must know how to walk on a leash early, to adapt to its daily walks. Puppies or dogs that don't know what it is can become overwhelmed or immobile by this new sensation, so you have to know how to unlock them.
We have seen puppies many times that the first time they are put on a leash they either wriggle or they stand still, because they don't like the feeling of someone pulling them, and they don't know that they have to walk alongside us. The more we throw, the more they will overwhelm and block, so we have to go little by little.
One of the best tricks for leash walking is bring some sweets with us. As soon as they get overwhelmed, we must stop applying pressure with the leash, and if they do not continue to walk, take out the candy to motivate them and continue walking. It's about associating leash and walking with something good, to make it a positive experience for them. Over time they will get used to the feeling of the collar and strap and they will already know that they should not be overwhelmed, that we only guide them with it.