This difference is key to the way living things store energy. When a cell has energy available, it can store small amounts of it by adding a phosphate group to ADP molecules, producing ATP. ATP is like a fully charged battery, ready to power the machinery of the cell while ADP is like a 3/4 charged battery
Energy from ATP powers other important events in the cell, including the synthesis of proteins and nucleic acids and responses to chemical signals at the cell surface. The energy from ATP can even be used to produce light.
ATP produces movement too, providing the energy for motor proteins that move organelles throughout the cell.