Yunio, the narrator of The Brief Wondrous Life of Oscar Wao, footnotes to establish ethos, or credibility and trust of the author, and pathos, past "suffering" of Dominicans. Because he is an outsider to the de Leon family (and to Trujillo's reign in the Dominican), Yunior's footnoting establishes him as a scholarly and outspoken voice.
Such marginalia once seemed bulky or too academic in texts, but Yunior's use of it is playful. He uses the second person familiar "you":
For those of you who missed your mandatory two seconds of Dominican history...
Yunior even drops the F-bomb on Trujillo, calling him "F**kface." Though this is anything but scholarly, it establishes Yunior's voice and Trujillo's character as mythical. Later, it will serve to characterize the "fuku," the curse that characterizes the cruel dictator.
Such footnoting is prevalent in our hypertext internet culture. Writing, more and more, has become linked to give it more context. Yunior, a product of the digital writing age, uses these footnotes much like a "blog," to give historical authenticity and to frame it from his own immigrant voice. Because the novel is a mix of Spanish and English, fantasy and reality, novel and comic book, superhero and villain, the footnotes are a way to blend the actual dictator with his mythical character in the book and a way to blend Yunior's scholarly voice with his younger "macho" voice.
Diaz's writing is full of esoteric science-fiction, comic book, and pop culture footnotes. In fact, there is a website devoted to explaining these references:
The author Kim says:
I was reading "The Brief Wondrous Life of Oscar Wao" by Junot Díaz, and it was extremely slow going since I need my laptop nearby the entire time, with Wikipedia, Google, and Google translate open. This was also annoying because it meant that I couldn't really read on the subway or elsewhere without an internet connection, unless I wanted to miss out on half of the story.
So, the novel is a kind of archipelago (series of disconnected islands), many voices all conflicted, meant to confuse us all--black and white, Dominican and American, Spanish-speaker and English-speaker. In this chaotic culture, we are all kind of immigrants.