The browning of meats and poultry (and sometimes vegetables) gives flavor to the meats and any stock in which they are to be cooked and, eventually, to any sauce made from those cooking liquids. While the meats are browning in fat, they are also being seared, so that when they are cooked later (as in braising), their juices are sealed inside.
Some points: the fat in which meat is seared and browned has to be removed from the pan before braising. So for fatty meats like short ribs, why use fat in the first place? Just season them and then brown and sear them on a rack in a 450-degree oven or under the broiler. After the meats are browned, do not put them on a flat surface like a pan or platter. If you do that, the seal is broken where the meat contacts the surface, and the juices leak out. Instead put them on a baking rack, and the juices that leak out are minimal. This tip is courtesy of Frank Stitt at the Highlands Bar & Grill in Birmingham, Alabama.