Why do pelicans have a sack in their beaks?

Pelicans use the sack for catching fish. Interestingly, different species of pelicans use different hunting methods. Some hunt in a semicircular formation and drive the fish from deep to shallow water so that the fish cannot dive down. The pelicans then immerse their long beaks in the water and scoop the fish in their sacks. The baby pelicans feed by taking the fish from the sack of the parents. Another species, the brown pelicans, does a vertical nosedive in the water from a height of 10-20 m and fills its sack with fishes from the deep water.