I was asked this at Sacred Revolution (VBF - Las Vegas) last night and wanted to put this in print for the benefit of others interested in this question.

It is true that if we were not to pray for some lost person then (if they are destined to be saved) they would be saved anyway. So this prompts the obvious question, What place then for praying for them?

Petitionary prayer is much more dynamic than the popular level view that God is some sort of "divine vending machine" where we insert our prayers and automatically receive action from God to serve that petition. However, it isn't so much to mobilize God to action (though sometimes such prayer serves that function - "you have not because you ask not") as much as it is to serve additional functions toward that end. For example (and I can only speculate since no one truly knows how our prayers for someone's salvation will matter), perhaps our praying for someone helps to reduce obstacles that otherwise may have impeded the progress of that person's salvation. Perhaps in praying for that person we expedite the timing of their salvation so that a sign is given to those who otherwise may have lost heart in the power of God. God is free to alter the circumstances that would otherwise slow down or depress the timing of one's salvation and perhaps God wants to honor those petitionary prayers by expediting the timing or reducing those obstacles that would impede that progress.

Petitionary prayer is a double-edges sword, too. It isn't always the target of that prayer that such prayer is meant to affect. It also serves to put believers in agreement with God that it is within His power and ability to grant someone's pathway to salvation. In other words, we are reminded that God draws all men unto himself and that without Him we could do nothing. Prayer secures our attitude that God is ultimately in control and it is a form of worship toward Him to recognize precisely who is responsible for that person's salvation. As philosopher Michael J. Murray notes,

"Not only does doing this reserve us from idolatry by forcing us to recognize that God is the ultimate source of all the goods we enjoy, it further ... fosters in us a genuine appreciation for the provisions that are made" (M. Murray, "Does Prayer Change Things?", p. 8)

The ultimate answer is not likely to be totally satisfying in that we expect our prayers to always bring about the desired action. It must be noted that in most cases God responds to our petitions. But that prayer is much more dynamic in that it serves to check the believer and to influence others who may be affirmatively affected by this action. And any opportunity to expedite someone's salvation is surely a benefit to the person who comes to know Jesus sooner rather than later!