The Berlin Airlift was an important factor in the struggle for Berlin largely because it showed the Soviet Union that the United States and its Western allies were very serious about holding onto Berlin.
The Berlin blockade came about as the climax of growing tensions between the USSR and the Western allies. The Soviets had been constantly harassing the Western allies as they brought supplies into West Berlin (which was surrounded by communist territory). When the Western allies introduced a single currency for their three zones of occupation in Germany (and insisted that it be used in West Berlin as well), the Soviets blockaded Berlin. The Soviets hoped that the blockade would force the Western allies to leave the city.
Instead, the blockade strengthened the resolve of the West. They did not want to be seen backing down in the face of Soviet aggression. Therefore, they instituted and maintained the Berlin Airlift. By doing this, they showed that they were deeply committed to holding Berlin. This show of commitment helped to perpetuate the division of the city of Berlin and made it clear that the West would not leave the city.