Flashback and flashforward are not a way of writing an entire story - that is more non-linear narrative - but rather a way of giving information as to what has happened in a story where the direct chronology would bog things down. This is especially true of the flashback; in a linear narrative a flashback would just extend the intro and could lose a reader. Introducing it as a memory or something like that can make the story flow much better. Flashforwards are used more in film, but do the same thing to stop the ending dragging out.

So, if a story starts with A meeting B and they become friends and then they have a falling out, and they lose contact... but the story itself doesn't start until A seeks B out because of reasons, then to stop the reader losing interest, start with A and B meeting after years apart, and use the flashback technique to show how they met, then another flashback later on to show why they had the falling out. It's just a way of stopping the backstory bogging down the introduction. So it's not strictly non-linear, just introducing backstory in dribs and drabs.

Sorry if I've overstepped the mark...