Duke decides he'd rather run off and play with the dummy than deliver, or he skirts around you inviting chase. You just throw to the end of the cable. He can't go farther away and he can't skirt past on a short slider.
On leash during initial heel training, it's a hassle to keep the pup from forging ahead, getting under foot or running too far out. But snap both cable tether and leash to the collar, and Duke stays in line with your heels and at your preferred distance.
Both "sit-stay" and "whoa" mean don't move, but pups want to break and come. They can't when the slider is taut against the cable end.
Right-handed retrievers tend to drift to the right; lefties tend to drift to the left. Through practice on cable, both learn to make straight-line runs.
Ground cables are especially useful in teaching doubles, triples and blind retrieves. Start with an in-sight group of three dummies. Spread them farther and farther apart until one is a close fetch and the farthest is hidden near the end behind a pile of grass. Accustomed to knowing dummies will be there, the dog will go for the blind with confidence. Also add decoys out of reach, so going through them won't tempt Duke on opening day.