One of the reasons is because we want to trap that glucose inside the cytoplasm. We want to keep that glucose from exiting our cell. And by adding this negatively charged phosphate group, we basically keep that glucose inside the cytoplasm because it no longer is capable of being transported across our cell membrane. So once our glucose is transformed into glucose phosphate, it stays within the cytoplasm of that cell. The second reason why we have to phosphorylate our glucose is to transform the glucose from the low energy stable form into a less stable form. So we destabilize the glucose by adding the phosphate group.