The current in the forward bias is known to be more (~ mA) than the current in the reverse bias (~ μA). What is the reason, then to operate the photodiode in reverse bias?

Hint: The p-type is connected to the negative terminal and the n-type side to positive to make a reverse bias, it is necessary in the low current flowing situations.

Complete step by step solution:
The forward bias is the process of connecting the p-type terminal to the positive side of the battery and the n-type terminal to the negative side of the battery. Whereas, inthe reverse bias the p-type terminal to the negative side of the battery and n-type to the positive side of the battery to flow the current.
The width of the depletion layer and capacitance increases in the reverse bias, to decrease such depletion layer and the capacitance in the reverse bias, a photodiode is operated in it that helps to operate more current than without a photodiode.

Note:
Be sure about the connections in the forward and reverse bias, in forward the p-type is connected to the positive terminal and n-type to the negative terminal of the battery and when it comes to the reverse bias, the p-type is connected to the negative and n-type to the positive terminal.