Most of the men left were 3rd and 4th line troops. They would be useless in the West, much like the Austro-Hungarians were OTL in 1918. However the major benefit to both would be relieved of the need to logistically support their far-flung occupation troops with their woefully inadequate resources. This won't change things much in the terms of a CP victory or draw, as the deal was sealed with US entry, but there are some major effects on the course that 1918 takes which can change the post-war situation.
First, the AH will freak out that they can't try to exploit Ukraine for food, but OTL it didn't supply anything for the home front anyway. Instead, it fed the troops on the ground, but at very high political cost, as the so-called 'bread peace' resulted in the AH turning parts of Galicia over to the Ukrainian nationalists for food, alienating the loyal Poles, which in turn helped to destabilize the empire as this once key group now joins the call for independence. Without occupying Ukraine, AH won't turn this land over and piss off the Poles, and may even get grain in the peace deal.
More importantly, without the promise of Ukrainian grain the AH are forced to start farming again, hopefully meaning they start to demobilize soldiers for farm labor and return some nitrates to agriculture. Hopefully this will enable troops returning from Russian PoW camps from turning to brigandage when they were denied this option OTL. Also, the weakest point of the AH war effort was their logistics meaning they were least able to send troops beyond their borders and supply them. Without the need to do that, trains can return to stabilizing the home front, preventing situation like in 1918 where 5000 train cars full of wheat rotted in Trieste because there wasn't enough locomotives to pull it, as there were too many trains being used in the East. So things are less unstable at home in 1918 (hopefully).
Same goes for the Germans-they won't have to supply troops in the Caucasus so those trains and coal can be used to transport food from the country side to the cities, helping morale. Now if they could only get rid of the incompetent Hindenburg plan....
1918 sees the CP home front and, by extension, the remaining fronts more stabile due to better supply. It also frees up crucial manpower for duties at home, which were not decisive but helpful nonetheless. The war is still lost, but with a more stabile home situation perhaps the CPs can then hold on longer, which would prevent a Versailles type situation, as the CPs can then reject it without worrying about revolution at home (well as bad as OTL) and push for moderation of terms.
Honestly the best thing the Habsburgs could do would have been to pull out of Northern Italy and the precarious supply situation there to they're own borders and defensible terrain with intact armies, enabling them to negotiate or force the Italians to fight their way into Austria, giving them leverage in a peace deal. It also helps change the way the Empire dissolves, perhaps changing the post war situation immensely, creating greater stability in the post war states.
Austria is the one state that stands to benefit, as with a stabile army and more stabile home front, they could defend their new state with their defensible borders. This also means that more German minorities near the border could be included in the nation by militarily means, which also helps support the Germans in their bid for a negotiated peace. It defends their southern border and could very well force the Allied forces moving through the Balkans to pause while pushing on, as the prospect of fighting through Austria is not exactly appealing. Austria can then be joined to Germany after the Habsburgs are removed from power by internal pressure and the Empire collapses. This sets precedent for these nations to be joined. Hungary is still screwed. The Czechs might now be able to get exactly what they want in the peace deal either, if it means Westerners having to die for their aggrandizement. The Galician Poles are a bit of a wild card, as they will probably revolt and join Poland, but will they be judged for staying with the Austrians for longer ITTL? Also, without the bad blood over the Ukraine deal, will the Galician Poles be up for invading Germany with the rest of the Poles? They are the breadbasket of the new state and have disproportionate power then in the new state.