This type of leash is a replacement for the standard leash, very useful for dogs that tend to chew and destroy their leashes. They?re available in various weights and thicknesses so are suitable for all sizes of dogs.
You do need to keep an eye on your dog if using a metal leash. The reason they?re usually bought is also the reason they can be problematic: Although most dogs soon learn a chain leash is indestructible and stop chewing it, some will continue to chew the lead, even to the point of damaging their teeth!