Why should a photodiode be operated at a reverse bias?

When, the photodiode is reverse biased then, the amount of electrons which is flowing through the p-n junction barrier is proportional to the amount of light illuminating the diode. And, as a result of the absorption of light, current is generated in the depletion region. This current which is generated is directly proportional to the intensity of light. 
Also, the change in saturation reverse current is linearly dependent on varying incident light flux which can be measured accurately. But, this is not the same condition when the diode is forward biased.