It brought matters to a conclusion faster than would otherwise have been the case (six months? A year?) and so saved a considerable number of lives, both on the Western Front and also in Germany where the impact of the blockade on the civilian population was considerable. By the time the American reinforcements actually arrived, the lunatic gamble that was the Ludendorff Offensive was already being pushed back and from that defeat there could be no recovery, but there's a reason why it was launched when it was in the first place.
Yes, the best interpretation of the US' role is that it put a shot clock on the Central Powers. The overwhelming numbers of American soldiers would make Germany's situation impossible once they arrived in large numbers, forcing Germany into a "throw absolutely everything into one giant last ditch gamble offensive" in March-May of 1918, which as Al noted Germany was doomed once it failed. No threat of US reinforcements and Germany might play a more cautious and sensible game following the withdrawal of Russia and take its time using the vast resources of the Brest-Litovsk concessions and the freed up manpower in more practical and sensible ways than all-out frontal assault.
