Slow cooking is naturally low in fat. Aside from some preliminary sautéing, most slow cooking uses very little oil. Typically recipes call for meats that have been trimmed of fat and skin; the meats themselves usually have less marbling than more popular, expensive cuts too. Slow cooker meals cook in their juices, not in fat, which makes them naturally lower in calories and rich in flavor.
Consider this too, especially if someone in your family is on a diet. Where fat often equals flavor, it comes at a high price. Slow cooked meals use substantially less fat than many fried and even baked dishes, but they're still bursting with flavor, vitamins and minerals. If you thought you had to give up flavor to lose weight, guess again.