Well, it is not as black and white as you might think. If I simplify it a lot, going from 200 pounds to 120 can prolong my life by 10 years I wouldn't mind losing 2 by doing it.

It doesn't just make a number go down, it leads to less strain on basically every component of your body. Muscles, bones, joints, organs, you name it. Lower blood pressure, better blood work, less strain on the system. High weight is the culprit for a lot of problems rather than just being a number and occupying more space.