Weight loss should _never_ be considered? How does that follow? Lots of people definitely need to lose weight.
Most studies look at more than just weight, they minimally assess BMI and ideally estimate body fat percentage. Some go further, looking at relative risk of other endpoints such as heart attack, heart failure, stroke, diabetes, injury, hospitalization, death, etc.
They start going to the gym; after some time, they'll have (random numbers) (X-2)% body fat, and (Y+2)% muscles.
On a scale, their weight will be the same, but in reality, their body composition (and looks) will be considerably different.
This is a very important concept. Beside the obvious case where somebody just looks at the scale, and they don't realize their progress, body composition as opposed to weight is extremely important when doing diets, because an improper diet may lead to excessive muscle loss, which is accidentally perceived as a success (on the scale) rather than a failure (because the body composition hasn't changed as intended).