I believe the answer is making sugar. The primary function of the calvin cycles is to make organic products the plants needs using the products from light stage of photosynthesis (ATP and NADPH). These products include glucose, the sugar made using carbon dioxide and water, and also protein (using nitrogen fixed from the soil) and lipids (fats and oils). Calvin cycle produces 3-carbon molecules which are then converted into hexoses (6 carbon) sugar, glucose.
