ATP has a position midway down the list of standard free energies of organophosphates. So ATP acts as a donor of high-energy phosphates to the compounds which have free energy less than the ATP. ADP can accept high-energy phosphate in the presence of enzyme to form ATP from the compounds whose free energy is greater than ATP. Hence, ATP/ADP cycle connects these two types of processes, that is, those which generate ~P and those which utilize ~P.