A fetal Doppler scan is important because it evaluates the blood flow in the fetus and placenta, which provides critical information about fetal well-being. By assessing blood flow through the umbilical artery, the Doppler scan can detect issues such as poor blood flow or signs of fetal distress, helping identify conditions like intrauterine growth restriction (IUGR) or placental insufficiency. This early detection allows for timely medical interventions to address potential problems and support a healthier pregnancy outcome.