Why does a resistor have this value in a circuit with an operational amplifier?
In theory it’s to balance out offset due to bias current so the resistance is equal to the parallel combination of R5 and R6. 49.9K is the closest value to 50K in the E96 series that many 1% resistors are manufactured in.
In practice, the voltage drop due to the entire bias current is less than 4uV at up to 85°C for the particular op-amp chosen so it won’t make much difference, given the offset voltage and offset voltage drift of that part.
A secondary reason to have a relatively high value resistance there is to protect the op-amp input against transients. With 50K, the 2mA maximum input current would allow a +/-100V transient safely.