The narrator in Like Water for Chocolate is an unnamed but presumably female distant niece of the novel's protagonist, Tita.  It is interesting and somewhat unusual for an author to tell a story from a first person point-of-view which is not a character directly involved in the action of the story.  This narrator is important for a couple of reasons.

Keep in mind that the story itself is told as a chronicle of family folklore.  The narrator is a modern voice telling a very personal story of the past. As a result, the unbelievable details in the effects of Tita's recipes, for example, (referred to stylistically as "magic realism") are not questioned but rather, accepted as natural and necessary elements to the folklore aspect of the story.  Additionally, a modern and culturally diverse audience is invited into a more personal connection to characters who could otherwise remain distant as a result of cultural differences.

Finally, when, at the very end of the story, it is finally revealed that Tita's recipes were found by the narrator's mother among the ruins of the De la Garza family ranch, it is as if this story of folklore might have stronger elements of truth than fiction.  Just as Tita's spirit will "go on living" through the recipes, her story will continue to be passed on through a maternal line.  Despite Tita's original conflict as the youngest daughter of an overbearing mother and a family tradition prohibiting her from marrying (and being remembered through her children), she is remembered, and celebrated, anyway.