What is the meaning of the flashback in Chimamanda Ngozi Adichie's novel Purple Hibiscus?

Chimamanda Ngozi Adichie moves the reader through time in two different ways in Purple Hibiscus. At the end, the reader realizes that the character of Kambili is looking back three years, to the event that directly led to her brother Jaja’s imprisonment—a period that is about to end.
At the novel’s beginning, Adichie alludes to the earlier famous Nigerian novel by Chinua Achebe, Things Fall Apart, as she offers the first words" “Things began to fall apart . . .” Because Achebe’s novel is about the downfall of a strong, proud man, the reader already starts to suspect that there will be tragedy in this book as well.
The first chapter is set on one fateful Palm Sunday. After Kambili, as the first-person narrator explains, what happened that day, she moves back in time to “before Palm Sunday” and then provides the full context of how the tragedy unfolded.
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