Bees make excellent pollinators because most of their life is spent collecting pollen, a source of protein that they feed to their developing offspring. When a bee lands on a flower, the hairs all over the bees' body attract pollen grains through electrostatic forces.
Basically plants have coevolved with these pollinating insects. Plants that can develop the best pollen that sticks to these insects are able to reproduce more successfully. While bees, as the example, are collecting it as there food. Bees are not "helping" the plant knowingly, its more of the plant is hitching a ride.