Can food only diet consists of 80 % water and does nothing for the dog.
It is true that compared by weight dry food supplies more nutrients, since canned, moist or fresh foods have a higher water content, but it is definitely incorrect that canned food "does nothing" for the dog.
As an example, if a dry food contains 10% moisture, that means 90% of it are dry matter. If a food contains 75% moisture, 25% of it are dry matter.
However, would you avoid eating veggies, fruits and other non-processed (or at least not highly processed) things as part of your daily diet, just because they contain water and thus might not contain as many nutrients per serving as some dried energy bar?
Dry foods rob the body of moisture, and many dogs do not make up for that sufficiently by drinking extra water. Canned foods have a consistency that is closer to the natural diet of a dog, are handled better by the body, are less processed than dry food, contain less additives and preservatives, and even of the mediocre brands of pet food, the canned ones often have better ingredients with less useless fillers than the dry varieties.
So if you go strictly by math, yes, you get a little less volume for your money when buying canned food, but your dog's health will definitely benefit.