Christians: If God's has a plan and nothing we do can change that plan then whats the point of prayer?

Often times Christians will say..."if it is your will" (I was raised a Christian so I know they do say this) which to me makes no sense because if its his will, then whatever he wants to do will be done anyway. This brings me back to my question, why do people pray for things (example: safe flight, healing a sick person, etc...). Also if you've read this far, why don't people pray for things that are in the realm of possibility? No one ever prays for a loved one to grow back a limb for example. Thanks for the answers ahead of time :)