Von Schlieffen plan was the name of the plan of the German invasion of France and Belgium in August 1914. It was based on the belief that in this war Germany would be faced with the war on 2 fronts ( France and Russia ). Also plan assumed that Russia would need 6 weeks to mobilize its troops. The plan imagined that the German army ( using 90% of the potentials ) will quickly took Paris. But in reality, Russia took only 10 days to mobilize and many German troops were sent to the eastern front.
Answer: According to the "von Schlieffen plan" it was imperative that Germans avoid weakening the western front before the fall of Paris.