“Want to know how to slim down and still eat the foods you love? Just go for fiber-rich whole foods and watch the pounds come off.” [...] “Trying to lose weight? Then fiber-rich foods definitely are the way to go. Case in point: Researchers at the University of Minnesota found that people who ate the most vegetables, fruits, and other fiber-rich foods lost 2 to 3 pounds more per month than those on lower-fiber diets.”
Basically, they explain why vegetarians and vegans tend to be slimmer than non-veg people, without trying to explain that: veg*ns tend to eat more unprocessed foods that are rich in fiber whereas non-veg people tend to eat more processed foods that are low in fiber.
But why lose weight? Because obesity is linked to cancer, diabetes, and heart disease.