There's a big difference between what you think of when you read the ingredients that go into most dog foods, and what the stuff inside that bag actually looks like. That's because those ingredients are cooked using a process called extrusion, which involves high heat and high pressure, in order to create a product which will last a long time on the shelf. This intense processing denatures the ingredients, destroying many of the vitamins and minerals naturally present, so manufacturers have to add those vitamins and minerals back in, usually in a synthetic form mixed in with a slurry of fat and preservatives sprayed on the food at the end of the process.
Many veterinarians and animal nutritionists have increasingly seen evidence that these synthetic nutrients aren't easily absorbed by our dogs' digestive systems, so instead of these vitamins and minerals benefiting our dogs, they often simply pass through the dog's digestion and end up in their waste. Additionally, the intense heat and pressures involved in these processes can cause chemical reactions which create potentially harmful byproducts which dog food manufacturers aren't required to list on the label, because these dangerous compounds occur as a result of the cooking process.
In short, your dog will get more nutrition and fewer potential toxins out of a dog diet which contains whole food ingredients - meaning, ingredients which are raw or only minimally processed, so that their nutritional content is still present in its natural state.