I was asked by a non-Muslim what the purpose of making dua to God is from the Muslim perspective. Essentially, he stated that he feels that since God has each member of humanity's destiny (kismat) preordained, our duas are either redundant (since if it is in God's plan for us, what we make dua for would happen regardless of whether or not we make dua) or futile (since nothing will happen unless it is in God's plan for us). What is the best way of responding to this question? What I have always been taught is that the relationship between our making dua and our destinies is something beyond our scope of human understanding, but I am posting this question here in the hopes of receiving more detailed responses rooted in Islamic sources.
JazakAllah Khayran for all of your help.