chicken broth is very nutritious because it has protein in the chicken, vitamins/minerals in the veg and has either a creamy broth packed with dairy/calcium or a flour thickened broth with carbs to compliment those in the potatoes!
chicken noodle soup is higher in calories but had more carbs too, both are good for gentle introduction for food in your tummy, also having lost weight it may be to build your body back up to an acceptable weight. they probably suggested chicken rather than beef or whatever because it is easier to digest.