The Curious Case of Benjamin Button contains a frame story of a woman reading a diary to her dying mother, as her dying mother fills in details. I found the frame relatively pointless: all it did was add unnecessary length to a film that was already too long. The narrative would have worked just fine without this frame. And if the frame appeared only at the beginning and end of the film, it would be a pretty inoffensive structure. But this frame constantly intrudes on the central narrative. The frequent interruptions add little or nothing to the narrative itself: they merely break it up.
I can see one possible purpose to the constantly intruding frame story. There are many things that don’t work in the film, and some things that do. One thing I like about the film is the defamiliarization. In many ways, The Curious Case of Benjamin Button shares stories of change and loss that are common in human experience. However, the conceit of the reverse-aging man defamiliarizes this experience, offers us a way to look at it anew. Perhaps the frame story, offering us a familiar image of change and loss, both adds to the defamiliarization of the central narrative and reminds us that, alas, the troubles of Mr. Button and his loved ones are truly universal.
I don't think this is quite enough to justify such frequent intrusions, however. The frame just didn't offer enough, broke up the narrative, and made the film far too long.