Superman being an alien is all the reason setting him in real world locations such as Smallville, Metropolis and the Southwest grounded him in reality, instead of looking like the Power Rangers, or Burton's garbage, where the artificiality is smothering and not at all how one should adapt a comic character to live action. Its on of the endless reasons Nolan's Batman films are classics, and the best portrayal of the character on screen: they have a costumed character, yes, but he--and his world are recognized by the audience as being close to their own, making any fantastic elements powerful, instead of a freakshow, or energy blast/explosion-of-the-month, like some comic films.