Put very simply, Photosynthesis is the manufacturing process for the synthesis of sugars. After manufacture they need to be 'packed', 'distributed' and made available to be 'consumed' this happens in the absence of light. When photosynthesis is not happening. So consider it this way, if you had fifty people working in a factory, during production you'd have as many people as possible working on that, production. Packing etc, would take place in the night, when raw material for manufacture isn't available. Now if manufacture went on and on, you wouldn't be able to get the product out and into the hands of the consumers in time. So you'd be doing a lot of damage, and stariving a lot of people... A very simple explanation.