What are the advantage for a flower to have its nectar located inside the flower rather then the outside?

The whole purpose of a flower producing nectar is to attract bees to it - for the purpose of having the bees help spread the flowers pollen to other flowers of its type. The nectar therefore must be in such a location as to have the bee dusted with pollen and have the dusted bee come in contact with the stigma of the flower. Nectar on the outside would not accomplish these vital actions.