It’s not required, because I lost 36 kg/80 lbs. as a couch potato, doing nothing but keeping carbs low. But while exercise can’t help very much with weight loss (according to several studies, anyway), it helps heal the mitchondria in the muscle cells and promotes the growth of new ones. This increases the metabolic rate and allows the body to burn more energy.
Also, if I’m not mistaken, it also helps promote mitochondrial uncoupling in the adipose tissue, which is when the fat cells metabolise more fatty acids than they strictly need, metabolically speaking (hence, the “meat sweats,” which should probably be called the “fat sweats”).