One thing I love about writing speculative fiction is you can amplify an aspect of the real world and it becomes metaphor. In my novel, Alien Invasion & Other Inconveniences, I was trying to use an alien invasion to explore ideas about corporate greed and colonialism and power and what happens when a much more powerful civilization meets a weaker one. We know, from our own past, what often happens. This time all of earth is on the short end of that stick. OK, I want to tell a story, too, and make the language sing and make interesting characters, but the metaphor—the power of comparison and the various shades of mythical memory they can inspire—opened up many opportunities.
The sequel of my alien duology is Homicidal Aliens & Other Inconveniences. This one plays more with myth, trying to use ideas of the “hero” to give the story and main character more emotional depth.
In Kristin Cashore’s Graceling series one of the evil characters can actually change the way people think. It’s his grace. Of course we see this kind of influence all the time--sometimes in the minor way of a dynamic speaker and sometimes in the more extreme way such as cult leaders like Manson and political tyrants like Hitler etc… She amplifies this kind of characteristic and uses the metaphor to give the story and character deeper value.
I think fantasy and sci-fi often are situational stories. There are great opportunities to use metaphor in any story but particularly in speculative fiction.