Wawa:
Maybe the designer was only interrested in fast light changes, not in static (sun)light.
Example is measuring the speed of a propellor of a remote controlled airplane.
Then you want to block static light (DC from the photo diode),
and only amplify the light changes from the propellor blades (AC ripple on top of the DC).
Another example is the pulses from a remote control.
You only want the remote control pulses, not the static light in the room.You might want to study capacitors and high-pass (or low-pass) filters,
so you understand at what frequency this blocking/passing occurs.
Leo..
Absolutely right,
This circuit used for a ballistic chronograph (calculate projectile speed). Projectile speed is high and so changing in light is too fast.
Thanks a lot Wawa