Fat is a way for the body to store excess energy. It is good because ounce for ounce, it contains the most energy, so it does not increase the weight of an animal as much as protein or carbohydrates. Fat is not just energy storage. Fat has also been used to insulate. Whales, dolphins and seals have a lot of fat under the skin to keep them well insulated. Fur does not insulate when it gets wet. The sea otter has a lot of fur and it insulates the animal, but the otter has to spend a lot of time grooming the fur, covering it with oil to keep it waterproof. Whales cannot groom themselves because the front legs have turned into flipper, and the rear legs have disappeared. Fur would not be useful, so they have evolved to be hairless and insulate themselves with subcutaneous fat.
Humans evolved on the open African savanna, and we evolved to be hairless too because it is hot and fur is not needed. Then 60,000-70,000 years ago, some African migrated out of Africa and some of them ended up in Europe 40,000 years ago, when it was the ice age. In order to adapt to the cold climate, the first Europeans had to evolve. Some of the adaptations include increased body and facial hair and more subcutaneous fat for better insulation, shorter arms and legs, thin lips and a rounded torso for reduced body surface area to conserve heat. Other adaptations include a taller and narrower nose bridge to capture more heat coming out of the lungs and returning it to the lungs with the incoming air.
So, for some humans adapted to cold climates, their fat is no longer just for energy storage. It is an adaptation to the cold. So, if these people were to experience a food shortage, then is it good for them to automatically lose body fat (which can result in frost bite or even freezing to death)? That may be why some people have bodies that will turn muscle mass into energy before fat is burned. That means when a person goes on a diet, he/she may not lose much fat, but will lose muscle instead. Since muscle burns energy, losing muscle means the body will burn less energy, making further weight loss harder. That may be why some people have so much trouble losing weight quickly. Starvation mode is not a myth. Scientists have found that animals that are underfed will lower their metabolism automatically to cope. A low metabolism also increases life span. That may be why animals that are put on a reduced calorie diet often live longer.