Just to elaborate a little, when the Germans came to the rescue in Greece, the majority of these units were poised to invade the Soviet Union. By diverting his forces away from the invasion of Russia, Hitler had to postpone Barbarrossa by a few months. This meant that the winter would arrive quiker, causing delays in troop movements, supply lines, etc, and this loss of momentum would give the Soviets time to build defences and regroup.