Aside from merely liberating France, the Allies were opening a long-awaited and prepared for Second Front in Europe. The fact that Hitler's Germany had conquered France and all of Western Europe meant that all he had to do was fortify the beaches and his entire Army would be free to concentrate on the invasion of the Soviet Union.
By successfully invading France, something that Stalin had been angrily demanding for two years already by June of 1944, Germany now had to divide its army and defend itself from two directions, an impossible task that could only lead to their defeat. This is exactly what happened only 11 months after the D-Day landings.
When the Allies invaded France on D-Day (June 6, 1944) their goal was to defeat Nazi Germany. The plan was to use the beach head at Normandy as a jumping off point. From there, the Allies would be able to drive into Germany from the West. As they did so, the Soviet Union's Red Army would have an easier time pushing into Germany from the East. Now Germany would be caught in a serious two front war and would be unable to resist effectively. This is, of course, what happened.
After the D-Day invasion succeeded, there was almost no way that Germany could have won the war.
The very obvious goal of Allied invasion of France was to liberate France from German Control and in this process reduce the power of Germany.
After the landing of their forces at Normandy beach in France and taking control of the beach, in the battle now famous by the name D-day, Allied forces were able to land one million troops there for the liberation of France from Germany and then advancing to Germany. Without controlling France, the allied forces would not have been able to establish a pipeline transporting their men and material for a war into Germany.