Plants need a constant exchange of air under their leaves to expire oxygen during the day and carbon dioxide during the dark hours. The faster you can exchange the air that is surrounding the leaves and the area that the plants are in the easier it is for the plant to perform this exchange. In addition as llamaman said, the rocking motion that a fan causes on the plants creates stress on the stem causing the plant to thicken and strengthen the stem. Hopefully the incredible number of buds on your plants will need a good strong stem for support!