Photodiode driver circuit
The photodiode has some parasitic capacitance. Its time constant is hence C times the load resistance. R2 plays the role of this load here; a sufficiently small value such that the response time will be acceptable for whatever application this circuit is intended. It also converts the output current into a voltage, thus giving the circuit a definite gain (200 V/A) independent of the load (within reason).

C2 also needs not to be fully charged if it is to work as a coupling capacitor, and R2 allows for that to happen by providing a d.c. path to ground.