Bee bread is a mixture of pollen and nectar/honey. It the main source of food for the workers and larvae. Pollen is stored in the cells and to stop it from going mouldy the bees cover it with a layer of nectar/honey. The pollen then can be unsealed by the bees when they need it. Late summer pollen is stored by the bees for use during the winter months and is needed in the spring to raise the new larvae. So this stored pollen is very important to the bees, especially when spring pollen may be in short supply.
Pollen is the sole source of amino acids in the honey bee diet and they need a variety of pollen types for a balanced diet. Not all the amino acids needed by honey bees are found in a single type of pollen. So a varied diet is vary important for colony nutrition and a lack of some types of amino acids could result in a shorter life span, poor foraging ability, less disease resistance and other effects on the colony.
Well fed colonies with a balanced diet are more able to deal with threats from
parasites, pathogens and pesticides. Honey taken from healthy colonies have been shown to have more antibacterial properties than honey taken from nutritionally deficient colonies. When bees have the full complement of amino acids, they are more able to secrete the enzymes that gives honey those properties, especially glucose oxidase. It is this enzyme that keeps the larvae free from disease.
As a Beekeeper you should be mindful of the pollen available to your bees as this provides protein, vitamins and trace elements needed in their diet.