Tougher, typically fattier cuts of meat (which are usually economical to purchase as well) make fantastic roasts when treated properly; that is, with a long, slow, low-temperature roasting treatment. The reason their meat is tougher and chewier is that it contains more connective tissue surrounding the muscle fibers.
The predominant protein in this connective tissue is collagen, which begins to unwind and break down into unctuous, moisture-filled gelatin only after the meat reaches an internal temperature of 140 degrees. So keeping the temperature of the meat in a range slightly above that (ideally 160 to 180 degrees) for an extended period gives the collagen all the time it needs to break down without danger of overcooking the meat.
Plus, the lower oven heat reduces the temperature difference between the exterior and interior of the meat, so you end up with more uniform cooking.