Why is photosynthesis divided into 2 different stages (light reactions and dark reactions)?

Photosynthesis is the process through which chlorophyll-containing plants consume carbon dioxide and produce oxygen and food (in the form of sugars). This process takes place in the presence of sunlight and can be summarized by the following chemical reaction:
6CO_2 + 6H_2O + sunlight -> C_6H_12O_6 + 6O_2
This process is made up of two different stages or phases and depends on light to carry out the associated chemical reactions. These two stages are described below: 
1) Light reactions: These reactions take place in the presence of light and result in the production of oxygen molecules, the ATP (adenosine triphosphate) molecules, and NADPH molecules. The ATP molecules are produced from the ADP (adenosine diphosphate) molecules and are the energy currency of the cell since they are used for storing and transferring energy in cells.
2) Dark reactions: These are light-independent reactions and result in the consumption of carbon dioxide molecules, the ATP molecules (generated during the light reactions), and the NADPH molecules to generate glucose molecules. The ATP molecules consumed in dark reactions are converted back to ADP molecules. Similarly, the NADPH molecules are converted back to NADP+ molecules which are then consumed in the light reactions.
Thus, the overall photosynthesis process is a sum total of the light and the dark reactions and each of these processes use the products of the other, thereby completing the photosynthesis process. Any of these stages (or phases) cannot survive for long without the other, and thus, they are interdependent. 
 
Hope this helps.