As far as I can understand your question, you wish to know why a plant cell consumes ATP to produce glucose when it can directly use the ATP as an energy molecule.
ATP is an energy currency and is required in different biochemical pathways. However, it is not a good energy storage molecule. Following are the reasons why production of an energy molecule such as glucose is essential:
- Not all parts of the plant are photosynthetic. These non-photosynthetic plants need an alternate source of energy. Since ATP is unstable, it cannot be transported to different parts of the plant without degradation.
- Since photosynthesis cannot happen in the dark, the plants would require some energy molecule that can be utilized later on, in the dark conditions.
- Chloroplasts themselves require ATP in the dark conditions. They express an ATP-ADP translocase that actually imports ATP from the cytosol while pumping out ADP+Pi
(See this review).
- Plants also need energy storage for seeds. This storage is usually in the form of starch (a polymer of glucose).
- Glucose is required for synthesis of cellulose which makes the cell wall of plants.
- The glycolytic pathway (including pentose phosphate pathway) produces important metabolites such as ribose (in the form of PRPP, which is important for nucleotide synthesis), pyruvate (which is used to synthesize alanine) etc.
- Glucose and some glycolytic metabolites (such as glycerol) also serve as osmolytes i.e. they maintain osmotic pressure in the cell.
Basically, points 4-7 convey that Calvin-Benson cycle not only produces sugar but what it actually does is fix inorganic carbon (as CO2) to organic form (in the form of sugar). So, most (practically all) of the carbon that a photosynthetic plant has, comes from this carbon fixation process and that's how plants are photoautotrophic.