Let's discuss aesthetics...
When attempting to build muscle, you need to consume a surplus of organic matter to grow. There are people who claim that so long as you consume the RDA of protein, it is irrelevant where you derive the remainder of your extra calories.
However, the RDA of calories alone is different for a 40 year-old, 160lb sedentary male vs. a 26 year-old, 240lb NFL lineman.
Also, consider the role protein plays in the anabolism process. An extra 1,000 calories derived from steak, eggs and fish will have a very different effect on building muscle than the same amount of calories from doughnuts and peanut butter fudge.