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?