Iraq will now have a government. While not the 'final' victory, it qualifies as a 'major' victory.
Simple, if we leave Iraq too soon and it is conquered by a unfriendly regime, as Iran or Al Queda they could then take many small ME countries like Kuwait, and the UAE that are friendly to the US. These countries are very rich because of their oil reserves and we are dependent on some. We import more than 60% of our oil supply. A oil supply disruption world wide would probably start a world war. We have 3% of the worlds oil but use 25% of the worlds supply. How many countries do you think we should fight at one time.
Iraq has a parlimentary system of government. Getting a government is a 3-step process. First, the Iraqi people vote for their representatives for a slate of candidates put forth by the party. Second, the party that gets the most votes designates its leader as Prime Minister, but he must be able to form a coalition if that party does not have an absolute majority -- this was the sticking point in Iraq. The Shia party that got the most votes failed to form a coalition, and the Prime Minister designate would not step aside. Now he has. Once the cabinet is formed, Parliment can meet (Step 3).