Brisket is the perfect meat for slow roasting. The fat around the outside melts into the meat, making it juicy, tender, and full of flavor. A 'bark' forms on the edge of the meat where it becomes crispy, creating those 'burnt ends' that are so sought after.
Wrapping your brisket joint helps to keep the juices close to the meat so you don't lose any flavor or moisture. This ensures that the meat is not dry or chewy. It also stops too much air from getting to the meat, which prevents the juice from evaporating in the heat of the smoker. This means that you can increase the temperature of the smoker without worrying about drying out the meat.
Another reason to wrap your brisket is for flavor. The wrap forms a barrier between the meat and the smoke, which means the joint takes on a smoky flavor without being overpowering. If the meat is exposed to too much smoke it will taste bitter.