To keep in the stored enzymes so it can keep synthesizing at night, even when there is no sunlight. -- Stomata serve two main functions - they allow carbon dioxide to reach the cells in the leaf where the majority of photosynthesis takes place and they allow water to be transpired out off the leaf. During the day, stomata are generally open to allow photosynthesis to take place as quickly as possible. At night, when the lack of light means photosynthesis can't take place, many plants close their stomata to reduce water loss. There is one group of mainly desert plants, known as CAM plants, that keep their stomata closed during the day when water loss would be greatest, and open them at night, using a biochemical method to effectively store carbon dioxide in the leaf overnight.
