Some one pointed out above that the President wanted to meet the Prime Minister in person to get to know him better and assess how he will do. GWB being such a good judge of character in people, now he has good idea of the sort of man he is dealing with. I think this is big bonus of him making the trip and that it will pay off big. Another important benefit of the trip is the impression his visit will have on the Iraqi government and the Iraqi people. Many of those government officials could listen to his speech in English without translation and they are bound to have been impressed.
I agree