Most reactions using ATP seem to involve:

ATP → ADP + Pi

but in some the reaction is

ATP → AMP + PPi

followed by hydrolysis of the pyrophosphate:

PPi → 2Pi

Is there any principle that determines which ATP-requiring reactions generate ADP and which generate AMP? And is there any reason why the generation of inorganic phosphate does not proceed as follows?

ATP → ADP + Pi

ADP → AMP + Pi