Doctors recommend heat for the hands as part of thermotherapy because heat improves blood circulation, which is essential for preventing and treating Raynaud's disease. This condition causes a temporary reduction in blood flow to the fingers and toes, leading to pain, numbness, and changes in skin color. Applying heat helps dilate blood vessels, promoting circulation and reducing episodes of vasoconstriction. Additionally, heat provides relief by improving blood flow and reducing stiffness in the joints, which is beneficial for both preventing and managing the symptoms of the syndrome.