Inverting Operational Amplifier for Dual Polarity References

I would like to use a voltage reference along with an op amp to get dual polarity voltage references similar to figure 44 in the image below. In this figure, there is a 5kohm resistor (R3) connecting the positive input of the op amp to ground. From my (somewhat limited) understanding of op amps, the currents at the positive and negative inputs to an op amp are negligibly small, meaning the voltage across R3 is negligible as well. Hence the gain would just be -(R2/R1) using node voltage equations which makes sense why the output is -5V given R1=R2 and the input voltage is +5V. In this case I don't understand why R3 is included in this circuit at all. Can someone please explain why resistor R3 is needed in this circuit?

Thanks,

Austin