By giving a warning of a rather large attack force (only Iraqi numbers actually given) you either scare the defenders into retreating or draw more in for the fight. In either case the US has an advantage. They flee and the city is out of their hands which turns it into a re-captured city. They dig in with larger numbers it's more in one place to attack, bomb and otherwise kill.
Both a retreat and or loss of thousands of ISIS mercenaries are also massive wins for the Allies as the rest of ISIS and other terror groups will see the loss and it should weaken their resolve.
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