The way in which flashbacks are used in this excellent novel is definitely something that is worthy of some serious attention. In particular, the author uses flashbacks to develop one of his central motifs, which is the relationship between fathers and their sons. This is of course primarily focused on the relationship between Baba and Amir, though of course other fathers and sons are mentioned in this text, including Hassan's own relationship with his boy.
To uncover one of the ways in which this is used, we can actually think about how Amir responds to Baba's death at his funeral. As he stands there and various people come up to him to express their condolences, he thinks the following thoughts:
Listening to them, I realised how much of who I was, what I was, had been defined by Baba and the marks he had left on people's lives. My whole life, I had been "Baba's son." Now he was gone. Baba couldn't show me the way anymore; I'd have to find it on my own.
The use of flashbacks is therefore one way in which Baba's influence lives on in Amir's life even after his death. Baba's example, where he has constantly challenged Amir to stand up for those who are unable to stand up for themselves, clearly impacts Amir as he comes to terms with his relationship with his father but only after his death. Flashbacks to key scenes of his life and his father's example shows Amir the way in which he must tread.