Why is an op-amp necessary for photo detecting circuit generally?
Both circuits are used and it depends on the application which of the two you should choose. There are two important differences between the modes the photodiode is used in those circuits:


The first mode is called photoconductive. It has a relative large reverse voltage across the diode which results in


low junction capacitance (good for high speed application)
high dark current (leakage current; bad for linearity) 

The second mode is called photovolatic. It has zero voltage across the diode (note: the polarity of the diode doesn't matter much; if you reverse the diode it will just reverse the polarity of the output voltage; but still works)


high junction capacitance (bad for high speed application)
no dark current, very good linearity (for quantitative measurements)



Photoconductive mode would be advantageous e.g. for digital data transmission.
Photovoltaic mode would be advatageous e.g. for measuring irradiance.