Just happened to read a couple good examples this weekend in the novel, A Thousand Splendid Suns, and thought I'd put them together. [Spoiler alert! Plot will be given away!]
While the flashback is most often about filling in backstory or providing crucial setup info, the flash forward seems to be about lending perspective and poignancy.
Splendid Suns is set in Kabul, Afghanistan, and centers around two main female characters, Mariam and Laila. In this scene, Laila's love interest, Tariq, has just revealed that he and his family are moving to Pakistan, to try to avoid the horrors of ongoing civil war. They make love for the first and last time, before Tariq leaves her forever. Afterwards, Laila tries to remember every detail of their last moments together, before we get a sudden flash forward into the very distant future, and a glimpse of how Laila will carry that memory with her:
This seems to have the effect, (1) of making this last encounter all the more poignant, by adding the perspective via the distant future, and (2) setting us up to believe that this parting is indeed forever, by making us feel that this future is quite distant, when (unknown to the reader) it is only a few years ahead, and not indicative of how Laila's entire future--in fact, as we'll see in a second, Tariq will return and undermine this assumption.
The second example comes later in the novel when--ta da!--Tariq shows up after years of separation. By this time, Laila has been forced to marry a brutal man named Rasheed, by whom she has a young son named Zalmai. Tariq shows up at Laila's home when Rasheed is at work, probably because he guesses--rightly--that Rasheed would violently force him out if he were home. Laila and Tariq's reunion comes at great risk, and the risk is shown to great effect by interspersing Laila and Tariq's real-time conversation with mini flashes forward into the future, in which Zalmai tattles on Laila to his father, Rasheed. Here's an example of one such cross cut between the present and the future. In the first few paragraphs, before the time break, Laila makes a bittersweet joke with Tariq about the "volumes" of letters she tried to send him in his absence. Cleverly, this reminds the audience of all the obstacles that have blocked the lovers' communications and connections, only to be interrupted by the next passage--Zalmai's revelation of his mother's secret meeting--which will prove to be yet another obstacle to their reunion.
The result of this intercutting (which happens several times) is extreme tension that ups the suspense level, and again, lends perspective to the present moment--while we've been rooting for Laila and Tariq, we have to acknowledge that there will be some grim consequences to their love, both for them and for Laila's son, Zalmai.
Quotes from: Hosseini, Khaled. A Thousand Splendid Suns. Riverhead Books: New York, NY; 2007.