Potatoes are very rich in starch, or slow-release sugars (19g carbohydrates per 100g cooked) and have all the 'good' qualities of carbs: they fill you up and release energy gradually into the bloodstream.
Potatoes are high in fibre (1.5g per 100g). Fibre slows down the digestion of sugars, so that the energy is released gradually over a long period of time. Fibre is also great for your intestines.
If you're looking to lose weight, look no further! Potatoes contain only 85 cal/100g, which is half the amount of calories - minimum - found in ordinary carbs (bread, pasta, rice etc).