How does the flashback to Abuela's life in Cuba affect the story?
It creates a mystery having to do with why Abuela and Alejandro didn't stay in touch after Abuela 's family left Cuba.
It sets up a surprise at the end of the story, where the reader learns that Alejandro, a boy Abuela loved in Cuba, has been writing the letters.
It provides a suspenseful subplot by showing how Abuela's family escaped from Cuba.
The flashback of the incident of young love between Abuela and Alejandro is the resolution to the story.