Roast what? But in addition to the earlier answers, in addition to cooking evenly it will also assist in carving serving sized portions. Also the center of the roast will be less well cooked, as in "rare". That is if you do it right...

If the roast is unevenly shaped, tying it will make it more uniform, which will help it cook more evenly. Some beef roasts, especially chuck roasts, are actually comprised of more than one piece of muscle, and may fall apart as cooked, so tying them will keep them in shape until you're ready to serve.

Sometimes to keep a stuffing inside. Otherwise to make a nice even shape.