There are numerous reasons for using the ATP to make glucose instead of just having the ATP around. the most important is that glucose can be stored in an insoluble form. ATP on the other hand is soluble. As you continue to make more and more of it, it builds up, the cell osmolarity increases and the cell swells. For plants this in itself isn’t a problem, because they have a cell wall to protect them from bursting. However, as osmolarity rises water potential also rises, and after a while the water potential for anything is so high that you can’t get anything inside the cell. This problem can be overcome by storing the energy in an insoluble form. Plant cells store a polymer of glucose (starch) for this reason.
Another important reason is that sometimes you need to export the energy: the ATP is made in the leaf cells, while energy is also required by root cells. If you simply had a pump that would pump ATP into the phloem then it would be hard to regulate how much ATP you export and how much you keep – ATP is ATP. Plants take care of this problem by turning the ATP for export into sucrose and that for keeping in the same cell into starch. This is done by a rather complicated mechanism of balancing the amount of phosphorylated trioses in the choloroplast vs the cytoplasm, but that’s the principle behind it.