They needed the deep water port at Cherbourg and the beaches at Normandy were located close to it. Grabbing a beach is one thing, supplying an army long term is quite another. The Mulberries were just a stop gap measure to allow supplies to be off loaded until they could capture the port at Cherbourg. I forget all of the strategic reasons but being on a peninsula they could also cut it off from the main German forces and prevent it from being reinforced during our attack on it. Once in our hands, the peninsula would prevent the Germans from launching a counterattack from any point around 180 degrees.
Remember that a good part of the plan for Hitler's last ditch attack resulting in the Battle of the Bulge was to take the port of Antwerp.
What's that saying - "amateurs discuss tactics, professionals discuss logistics."
