Knowledge of bodyfat weight is an important tool, even though the methods of determining it are imprecise (calipers, body-fat-measuring scales). The situation is more critical when an exerciser has a noticeable portion of his or her bodyweight as body fat rather than muscle. If an exerciser has a large portion of their bodyweight as muscle, use of bodyweight as a guide will more likely than not lead to success.
The goal of gym-goers looking to build up their physiques is to add muscular weight. Nutrition plays a role but only if you know whether the caloric intake is contributing mostly to muscle gain or fat gain.
Going by scale weight alone in inaccurate because it does not reveal the muscle-fat proportion of that weight.
For example, there are 2 people who weigh 225 pounds. But one has 25% body fat, and the other has 9% body fat. The physical look of each body will be quite different due to differing amounts of fat-free mass. The person with 9% body fat would be better off using total bodyweight, while the person at 25% body fat may be better off using the fat-free mass as a starting point.
225 lbs-56.25 lbs body fat=168.75 lbs fat-free mass.
225 lbs.-20.25 lbs body fat=204.75 lbs fat-free mass. [This person is likely athletic appearing or a muscle model.]