My conjecture is that "Changing Body Composition" is a better goal than "Lose Weight" for individuals who are trying to become healthier. As a long term aim or as a population measure, "Lose Weight" or "Lower BMI" fine; however, as a personal goal "Lose Weight" can be demotivating and flat out counterproductive, especially in the short term. It just doesn't work for many many people.

If you had a magic wand and changed all of an obese person's fat mass into muscle mass, that would not produce a healthy person. But that doesn't happen in the real world.

Here's what happens in the real world - when you start a workout program you gain a little weight. That small weight gain is not bad, but it can be demotivating if your goal is "lose weight". It can stop people from exercising, which is good for long term health.

Weight fluctuations are also confounding to the goal of "Lose Weight". I've often heard that a good goal is to lose 2 pounds a week. I have several problems with that. I can lose up to 4 pounds on a long walk. I can gain up to 5 pounds overnight. Neither of those reflect body fat loss (the walk may represent a few ounces). You may say "That's just water weight!" - Ok. How does my scale know that?

Being specific is very important in the personal experience of weight loss ahem change of body composition.