What is the difference between dogcat food and dogcat treats?

Why are there dog/cat treats that can only be given to pets a few times a day? Are they fattening? I was looking at some of the ingredients of a premium pet food and I was thinking: If pets love their food so much (assuming they do), couldnt you just give them a grain of dog/cat food instead of a dog treat as a reward? Both have meat-like stuff in it, both satisfy the pet, and isnt the pet food better for them? Is there some specific component in dog/cat treats that make them so much more appealing than pet food AND makes them so bad for them that they can only be given treats a few times a day?