Sigh, let me give you an example to why line breeding can play an important role in breeding programs...
So with the Doberman there is a bleeding disorder not unlike hemophilia, call Von Willebrands Disease. All breeding dogs are suppose to be checked for this before being bred. DNA tests marks the dog as one of three results. Clear, Carrier or Affected.
vWD status of the litter when Breeding dobermans
Now how would you think it was best to wipe out vWD? breeding only clear to clear right? Well, we can't because the gene pool would be too small. We are forced to breed to carriers for the most part. (Affected dogs are never bred from.) But when a "healthy" (free of heart issues) clear factor line is found it is used a lot. But breeders only go as far as line breeding. NOT inbreeding. And then those litters are out crossed with other clear lines, and the results of that litter are reintroduced back into the original lines. It's the only way to be rid of vWD. There are just not enough clear dogs unrelated. And breeding carrierxcarrier is not helping anything as the result is always 25% affected puppies. We have to have clear dogs to breed to.