You don't have to do it, but it's good to learn how. And with a little practice, it's very quick and easy to do. The bird cooks more evenly. The bird stays moist without adding fat. The final presentation is prettier.
Carving is easier.
With the breast side up, bend the second joint of the wings back so that they rest behind the top of the bird. Take a piece of butcher's string long enough to wrap around the bird twice. Tie it around the tail and leave the same amount of string on either side. Cross each piece of string over and around the drumsticks. Run each piece of string snugly along the breasts and thread through the hole made on each side by the folded wing.