As a dog trainer, I can tell you that most people have the goal of having a dog who’s obedient when they’re off-leash, but since they train their dogs using a tight/short leash the dog learns to depend on the feedback from the leash. This dependence makes a transition to off-leash obedience very difficult. When you use a long lead (that simply drags on the ground behind your dog as a safety mechanism for the moments when your dog decides to take off), your dog learns how to do things such as coming when called and heeling without the restriction of a leash from the get-go. So you’re actually training them to do things “off-leash” from the very beginning! The transition back to “on-lead” (i.e. with you actually holding the leash) is much easier than the other way around.