Here's why:

* ATP has three phosphate groups, while ADP has only two. The bonds between these phosphate groups are high-energy bonds.

* The breaking of these phosphate bonds releases energy. When one phosphate group is removed from ATP to form ADP, a significant amount of energy is released, which can be used by cells to perform various functions.

Therefore, ATP is considered the "energy currency" of cells because it stores and releases energy efficiently.