My family's dog isn't all that well-behaved. Keep him on a 6 foot leash and he pulls like mad. Allow him 15 feet of leash and he will be at the point where he doesn't really want to go further from his "pack". This is only useful when we're in a place with no foot or vehicle traffic so that's when we use the leash as a long leash.

And then on the bike path, 6 feet of leash is too much. Being able to shorten the leash to a few inches is nice, (and that's what we do) because then he can be forced to stay out of the oncoming path traffic.