There are significant advantages for a pet food company of Purina's size.
ownership of the manufacturing facilities
ownership of testing labs
performing feeding trials
huge clout with ingredient suppliers
These advantages can lead to better quality pet foods, if the company is pushed by consumers and competitors to provide the desired quality improvements
Brands like Party Animal Pet Food (pentobarbital recall) are at the mercy of the contract manufactures they hire and have very little clout with ingredient suppliers or are at thee mercy of the contract manufacturers if the manufacturer qualifies & secures the ingredients.