Resistance training helps with fat loss in a number of ways. Weight training itself burns calories. Studies also show that, unlike aerobic exercise, weight training increases the calories you burn at rest for up to 39 hours after your workout.
Plus, the more muscle your body has, the more calories you burn each day.
Even if your goal is solely to lose body fat, you need to train with weights. This will help prevent any of the weight you lose from being muscle.
Were that to happen, your metabolism would slow, stalling your fat-loss efforts and turning you into a skinny-fat person.
Yes, even someone with anorexia can have a high body fat percentage.