Mostly as a psychological and moral boost for the Allies. American soldiers only started to arrive in mid-1918 when the French and British already had started to finally force the Germans to retreat.
Germany's defeat was in a large measure caused by its war economy no longer being able to support the cost and demands of its war effort. Also, Germany's ability to replace its human losses or relieve and replace battle-weary frontline soldiers had run out by then.
America's soldiers only fought in any significant numbers in the very last months of the war. The psychological effect on the Germans was mainly the realization by their High Command that the Allies' capacity to carry on was now becoming infinitely bigger than their own.