Resistor Between Voltage Reference and + Input of Op-Amp? Why?
Figure 1. Both op-amp inputs have a bias current. If R3 is zero ohms then there will be a difference in the offsets between the two inputs. Source: ECircuitCenter.

From the same article:


  One of the golden rules of op amp analysis says this: no current flows into either input terminal. This concept is key for analyzing an amplifier's signal gain. However, in reality, a small current flows into both inputs to bias the input transistors. Unfortunately, this bias current gets converted into a voltage by the circuit's local resistors and amplified right along with the signal. The result is an output error in your circuit. What can you do about it? A clever choice of resistor values can help you cancel most of the output error. The remaining error can be adjusted to zero if necessary.