#1. The goal is to restrict caloric intake to slow growth rate. Studies have found that for breeds prone to get hip dysplasia a restricted caloric intake reduces the incidence rate of cHD. One way is to feed adult food another is to feed less puppy food; you want the pups to be thin not plump. I forget what age we switch our pups to adult food.
I suspect the grain free hype stems from a higher rate of grain allergies in breeds now because of higher rates of inbreeding and passing on of the genes responsible for these allergies.