A lot of plants only provide pollen and not nectar, Crepe Myrtle being one of them. The have no Nectaries. The bees just get pollen from them.
Some plants only provide nectar when they get the proper rainfall. Hence a good crop year/bad crop year.
Bees need nectar for fuel. They need pollen to feed larvae.
Bees can make honey and wax with no pollen but they cannot raise young without it.
Jim