Peanut butter (peanuts are actually legumes, not nuts) provides both monosaturated fats and protein to your diet. Protein you know all about. Monosaturated fats not only increase your HDL (good) cholesterol, but they are also essentail for distributing fat soluble vitamins throughout your body. A healthy diet should consist of between 20% and 30% fat, with the majority of that in the form of monosaturated and polyunsaturated fats.
Now, the reason for eating "natural" peanut butter as opposed to regular, commercial peanut butter is the conents: Peanuts, salt. that's it.
Commercial peanut butters such as Skippy, Peter Pan and JIF, on the other hand, contain not only peanuts and salt, but also sugar and hydrogenated vegetable oil (read transfatty acid). That is why natural peanut butter will separate at room temperature and the major brands don't.