The flashbacks serve several important functions in the novel. One of them is to provide relevant background information on the character of Winston; it helps to flesh out his character, and it helps the reader to feel a more personal and invested interest in who he is. When we learn about his family, his mother and sister, and their lives together, we can understand more fully where Winston is coming from. We understand his character and personality more, and it makes him more of a rounded, complete character. The more you know about a person, the more you tend to be invested in them. So, that is one purpose, to help us to learn more about Winston, and become involved in his character.
A second function of the flashbacks is to show the historical progression of Winston's society. We learn terrible, horrible truths: children were starving, food was nearly impossible to get to, and the government took people and children away at will. We underestand more fully how powerful and awful their government was, and how it used starvation to control and manipulate the masses, and to induce selifshness and a lack of caring in families. These flashbacks help us to see the longevity and destruction that the government possesses.
One last purpose of the flashbacks is to help us feel a brief respite from the dreary hopelessness of the current situation in the novel. One flashback was pleasant, that of him playing Snakes and Ladders with his mother. Winston remembers this at the very end of the book, after he has been "converted" to the Party. The memory is a happy one, and one of the brief glimpses of happiness that he has had in his life. It is a relief to learn about it. Unfortunately, Winston, at that time, dismisses it as "a false memory." This shows how completely the Party ahs gotten to him--he can't even cherish or trust his happy memories anymore. I hope that those thoughts helped a bit; good luck!