Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
"What's past is prologue" (The Tempest, Act II, scene i)
The characters in this novel are obsessed with the past. Ob. Sessed. Even Irie, who is often annoyed with Archie and Samad for talking endlessly about WWII, goes looking for a fuller picture of the past. So, why do the characters in White Teeth care so much about what's already happened?
As the epigraph suggests, the past is really like a prologue for what's going on now. The past informs the present; it gets us ready for what's coming. What would the characters in White Teeth be like without their pasts? We're guessing they'd be something like amnesia patients in a soap opera, and we think they're way more interesting the way they are.