The second power is important because the popular title of the law that established the interstate highway system was:
National Interstate and Defense Highways Act From the source at the link:
Eisenhower had first realized the value of good highways in 1919, when he participated in the U.S. Army's first transcontinental motor convoy from Washington, DC, to San Francisco. Again, during World War II, Eisenhower saw the German advantage that resulted from their autobahn highway network, and he also noted the enhanced mobility of the Allies, on those same highways, when they fought their way into Germany. These experiences significantly shaped Eisenhower's views on highways and their role in national defense. During his State of the Union Address on January 7, 1954, Eisenhower made it clear that he was ready to turn his attention to the nation's highway problems. He considered it important to "protect the vital interest of every citizen in a safe and adequate highway system."