Bees, while collecting nectar from flowers, transfer the pollen grains. Beehives are kept in a crop field during flowering period to enhance the pollination of the crop, which increases the crop yield. Also, bees can easily collect huge amounts of nectar from the flowers of the crop in a close reach without much foraging. This in turn increases the production of honey.
e.g. this technique is practised in apple and mustard fields.