Why is negative voltage bias important for my photo diode?
Reverse bias puts a photodiode into photoconductive mode, as opposed to photovoltaic mode.  This results in increased sensitivity, decreased capacitance, and increased speed.  This is at the expense of increased dark current. 

Good review at https://www.thorlabs.com/tutorials.cfm?tabID=31760

The best review I know of is the Sharp Application Note on Optoelectronics.  I'm having trouble finding it on the Shape site, but one link is http://denethor.wlu.ca/pc300/projects/sensors/photdiod.pdf

In particular, that Sharp AN shows the voltage/current relationships as a function of the reverse bias.  

FWIW, I'd recommend an active amplifier circuit for your photodiode just on general principles, but I have no idea what your application is.