Dogs, like all animals, need fats to survive and as carnivores, they are best suited to animal fats. Specified animal fats (ones where the animal species is named - for example, chicken fat, duck oil etc.) are particularly good as they ensure the recipe remains the same from batch to batch.
In general, animal fats are produced as a by-product of meat meal production. When the meat is heated and ground, the fats melt and are drained away. The resulting dry meat meal powder is therefore relatively low in fat so most dog foods that use meat meal have added fat to bring the levels back up.
Dogs also really like the taste of fat so it is tempting for manufacturers to add a little more than is strictly needed to make their foods that little bit more appealing. Unfortunately, as we all know, too much fat can cause all sorts of problems so if you notice your dog is starting to put on a little too much weight, you may want to consider a food with a lower fat content.