Exercise is not necessary for fat loss. It has many other benefits, but that is not one. A well-formulated ketogenic diet that provides sufficient calories and avoids inflammatory polyunsaturated fatty acids (in other words the fat in the diet should be mostly saturated and monounsaturated fats) will promote fat loss quite apart from exercise. If I, a notorious couch potato, could lose 60-80 pounds with no exercise whatsoever, that shows that exercise is not necessary to losing fat.
The benefits of exercise include the promotion of mitochondrial (and thus metabolic) health, muscle strength and growth, and bone-building, among others. Just not fat loss.