Most insect-pollinated flowers produce nectar and are sweet-smelling. Why?

Most insect-pollinated flowers produce nectar and are sweet-smelling to attract insects. Insects such as bees and butterflies visit different flowers for their food (nectar) and in this process pollen grains get stuck to their body. When they visit another flower, the pollen grains get transferred to the stigma of flowers. Thus, to facilitate pollination by insects (entomophily), most flowers produce nectar and are sweet-smelling.