Prayer

What is the reason to pray for something when God's will is going to be the final result anyways? For example, when someone gets diagnosed with terminal cancer or suffers great trauma in a car accident or something, people will always ask for prayers for healing and such. I have always been told that God has a plan for me and he has had it before I was even born into existence. So if God's plan is for me to die in a car accident or from cancer then what is the purpose for people to pray to God for him to heal me? Surely God is not going to change his will just because some people prayed to him and asked him to do so because if that was the case ( which I don't believe it is ) why does it work for some people and not others? Did not enough people pray for the sick one? I have also heard people say to pray for God's will to be done....why is that even necessary? God doesn't need our help to carry out his will.