To understand why commercial dog food may not always be the best thing to feed your dog, I think it's useful to know why it exists in the first place. There's really only two reasons - commercial and circumstantial.
The first history lesson takes us back to 1859 when an American salesman called James Spratt, who was working in London, watched sailors throwing their unwanted hard tack biscuits to some hungry stray dogs. He had a light bulb moment and started producing a biscuit that was specifically made for dogs. Other products followed and soon they were sold all over the world and the dog food industry was born. Spratt's was very well marketed and convinced a public used to feeding their dogs scraps, meat and bones that what dogs really needed were biscuits.