I'm not exactly sure what you mean by "purpose". The fuku itself can be defined most closely as a curse, so I guess the purpose of a fuku would simply be to doom a person/family/country to death or bad fortune.
The purpose of the fuku in the text of Oscar Wao, however, is a whole different matter. The fuku is present in the actual history of the Dominican Republic, as well as in the history of Oscar's family.
If you are doing a longer assignment and want to identify/support the purpose of the fuku in the text, examine the ways in which it affects individual characters. How does the fuku influence the character's lives (and by extension the novel's plot)? Ultimately, what does the fuku add to the story? What message might Diaz be conveying to the reader through his discussion of the fuku?
I think the purpose of fuku as intended by the author of the story is to help build on the element of the supernatural and to foster the reader’s belief in fuku. Fuku americanas is a curse believed to have originated from the New World and was shipped to the Caribbean Islands by Europeans. According to the narrator, an autocratic leader, Rafael Trujillo also has a hand in the curse that has overwhelmed the inhabitants of the island for many years. In fact, every family has been afflicted by fuku and in Oscar Wao’s family, the misfortunes of fuku follow them through until his demise in adulthood. There is a series of supernatural occurrences throughout the book. The author is trying to state that our lives are controlled by supernatural forces for which we have no control over except supernaturally. The concept of zafa is brought out as a way of attempting to ward off fuku’s misfortunes like in the case of La Inca when she prayed for her girl’s life. Therefore, the purpose of fuku is to help bring out the concept of the supernatural and to strengthen the reader’s belief in the curse.