Most farmers are careful to water the roots of their plants, understanding that the water is what allows nutrients to move from the soil into the rootlets. However, few farmers have yet learned the lesson that water vapor is as important to leaves as liquid water is to roots.
Raising the humidity allows plants to tolerate extraordinary heat and cold: in extraordinary heat and cold, it is not so much the temperature which kills the plant, but the dehydration.
With high humidity, plants are able to better exchange carbon dioxide from the atmosphere for oxygen, and create sugar. This helps the plant grow at an incredible rate.