Why Should We Pray If God Already Knows What’s Going to Happen?

Questions about why we should pray if God already knows what’s going to happen, how the effectiveness of prayer is measured, and whether or not things would have happened the way they did if no one had prayed for them.  

• Why should we pray if God already knows what’s going to happen?
• How is the effectiveness of prayer measured—e.g., would Christopher Yuan have been saved if his mother hadn’t prayed fervently?