The towering hill of Monte Cassino topped by the monastery was the linchpin of the line that the Allies were determined to capture, because breaking through the Gustav Line was necessary for reaching Rome, which was finally captured on June 5.
The Battle of Monte Cassino was significant because the Allied troops could push forward to Rome if the hill was successfully taken. It also helped to draw Germans away from the Soviet Union and distract them while D-Day plans were being finalized.