What about the structure of atp makes it useful as the energy currency of cells?

It has a high energy phosphate bonds, due to the large amount of electrorepulsive force between the highly negative force between the highly negatively charged phosphate groups. Energy is usually liberated from the ATP molecule to do work in the cell by a reaction that removes one of the phosphate-oxygen groups, leaving adenosine diphosphate (ADP). When this happens, the ATP is said to be spent. 

The hawk receives 0.1% energy from the caterpillar although the caterpillar gets eaten by another bird smaller than the hawk first