Voltage Range Mapping/Conversion from [-12V, +12V] to [0V, 4V]
R6 and R7 are there to compensate for the input bias current. If you had an ideal op-amp it would have an input current of zero and you could connect the pin directly to 0V, but in the real world their inputs do require a very small amount of current. This current flows through the input resistors and therefore produces a small offset voltage at the inverting input.

By connecting the non-inverting input through a resistor as well, you ensure that a similar offset voltage is seen at both inputs and therefore its effect on the op-amp output is massively reduced.