Tradition and quite a few successful chefs insist that one must frequently baste roasting poultry with fat in order to add flavor, keep the meat moist and the skin crispy.
Others assert that basting does the opposite, because it lowers the effective temperature when the oven door is opened each time and it does nothing for flavor, moistness or skin quality.
The usefulness of basting depends on what you baste with. A water-based baste can be helpful, for the same reason that repeated door-openings can be: slowing the heating means that the breast meat in particular cooks more gradually and gently, so you have a larger window of time in which to pull it before it gets dry. At the same time, it re-moistens the skin and so prevents it from crisping. Basting can certainly contribute flavor—I like to include a little butter for exactly that reason. Basting with oil alone—clarified butter for example—doesn’t contribute to evaporative cooling and so doesn’t gentle the heating in the same way.