Superman came around following World War I and the Great Depression, a time in which Americans were dealing with horrible circumstances. It is during this time that Superman had the perfect entrance; people needed a hero, someone to make their problems disappear for even just a moment.
Also at this time, there was a portion of the Work Projects Administration that was dedicated to the arts: the Federal Arts Project. Part of the Federal Arts Project was dedicated to artists that did comics. Later, as the nation approached World War II, those artists were used to create propaganda. By the time that was necessary, people were hooked by the comics they were reading.
Superman was an All American hero who represented everything the government wished its citizens to covet; physical fitness, self supported, a problem solver, and a Robin Hood of the people.
Now America is at war with the Middle East (though I cannot really fathom why we are still at it, but that is beside the point) and both D.C. and Marvel comics are taking their 'old' comics and making them into movies; Watchmen, Xmen, Spiderman, the Hulk, Ironman, and so many others. The same principle applies now as it did in the Great Depression; we need escapism and hero stories to help us deal with the terrors of war and the economy.