Storytellers frame their morals and fables of us through the lens of the 'Others'. When the popular form of storytelling evolved into film, the Other naturally became that which was most fascinating and curious, the Intergalactic Entity, or Alien.
In the film Contact(1997), Dr. Ellie Arroway struggles with a disbelief in things greater than herself, and the desire to see what is out there in the cosmos. In Close Encounters of the Third Kind (1977), strangers are brought together by the vision of harmony set forth by cosmic visitors. The theme of belief in something greater than oneself echoes through many different science fiction films of the 20th century.
In Alien Nation (1986), we have a group of refugees landing on earth in search of new life. In Alive in Joburg (2005), we have aliens as second class workers. All throughout the 20th century, aliens have been a viable mechanism with which to portray our views, dim and otherwise, of the 'Other'.
In the 1990's, Spike Lee, in an interview with Cineaste (1999), complained that blacks on film were rapidly becoming 'magical helpers' who assisted the white protagonists in reaching their goals. A similar fate struck aliens in such films as Cocoon (1985) and ET: the Extraterrestrial(1982).
Culturally, the Alien has been represented as both a work of horror to be terrified by, as in Alien (1979), and a romantic lead worth falling for, as in My Stepmother is an Alien (1986). Altogether, the purpose of the aliens in these films is very clear; to represent the best and worst qualities of our civilization, and wrap great stories around them.