To understand why your dog's food is so important to its overall wellbeing, you have to have a general idea of how your dog 'works'. Fortunately, while there are some differences, it's not too different to the way you or I function. There are a few basics to remember.
The liver performs some of the most vital functions of the body. It purifies the blood of toxins, and regulates digestion by producing bile. But it does something even more important. Inside the liver is an incredibly, jaw-droppingly complex system called the Cytochrome p450 System. Its many enzymes break down foods and ingested matter, and use them to keep our brains functioning properly. Essentially the liver is like the software centre of our bodies, the food we take in is the data - and as with all machines, the formula: "garbage in, garbage out" applies.
But it is not just the food we eat that regulates how our brains function; if the hardware, or the intestinal system, that delivers the data is damaged, just like a computer, the output will also be corrupted, and your machine starts malfunctioning.
The digestive tract is where nutrients and essential elements from your food are extracted and absorbed into the body. Each item of food you put into your body is a nutrient block of data that is processed through the gut, and ultimately goes into making your body run well...or not so well. This applies to every aspect of your health, from your mental & emotional, through to your physical wellbeing. But if the conduits that deliver the essential ingredients are damaged, or full of holes, then your body will start to experience run-time errors.
In a modern age, a lot of what we ingest does damage to our intestinal system. Food additives, preservatives, processed sugars, high concentrations of salt, pesticides, antibiotics, refined grains & flours - all of these and more can damage the intestine. A damaged intestine leads to the proliferation of bad bacteria and results in inflammation. Research in immunology has indicated that in addition to affecting the localised area, damaged intestines and 'leaky gut' can prompt autoimmune responses through the whole body:
"...leaky gut allows the entry of exterior antigens from the gut lumen into the host, which may promote both local and systemic immune responses. Multiple diseases may arise or be exacerbated due to a leaky gut, including autoimmune diseases such as inflammatory bowel disease, celiac disease, autoimmune hepatitis, type 1 diabetes (T1D), multiple sclerosis, and systemic lupus erythematosus (SLE) (2–6)."
The truth about standard, commercially produced dog food is not good. The quality of product that goes into dog food is allowed to be low, and much of the corn or grain fillers that bulk out dry food are unhealthy additions to your dog's diet, and will cause intestinal discomfort and damage over time. Added to this, your dog needs as much fresh food and variety in its diet as you do, so repeatedly feeding it the same thing out of a bag or a can day after day, is not doing it any favours. You will keep it alive this way, but you will not be contributing to its health and wellbeing.
For your dog, as with yourself, there are ways to repair and undo a lot of the damage caused by bad foods and environmental toxins. The first of these is by changing your dog's diet and making sure it has plenty of fresh, homemade food full of variety and balance. If your vet is unsure as to the best way to approach a homemade diet for your dog, then google has a lot of good advice, just make sure your source is reputable.
For the more targeted healing of already damaged intestines, there are herbs & supplements that will assist.
Preparing a fresh diet for your dog does mean going to more effort, but much of it can be done off the back of your own food preparation, and there are food products out there that offer all the right stuff to your dog, while taking some of the labor out of the task.
In the end it's worth it, because it means that our dearest companions have a greater quality of life, and hopefully a longer one, by our sides.