The major impetus for building the interstate highway system was Dwight Eisenhower's cross-country road trip with an Army convoy in the early 1920s, testing the movement of military material by road convoy, using the roads available. Subsequently, one of Eisenhower's specialties as a general staff officer became mobilization in time of war or increased readiness. Observing the German autobahn and similar European roadways in the mid-1940s provided a little extra inspiration, and when Eisenhower got into a position where he could do something about it, it became one of his pet projects in his presidential administration.
It's come to be used for many other things since the 1950s. but the original idea was indeed speeding up transcontinental movement of military forces and supplies... or at least one of the major justifications for using federal funds to pay for it...