What is the purpose of R1, R2 in this ESD protection circuit
Those buffer the protected pin from the ESD event while ESD diodes do their job since it takes a finite amount of time for them to do so. It is similar to placing the ESD diodes as close to the connector as possible and as far away from protected pin as possible which uses the parasitic inductance of the traces instead of a resistor. Ferrite beads can also be used.

One way to look at it is that they limit the ESD current that flows into the pin (and then through the internal ESDs or rail clamp diodes or capacitances) due to the ESD diodes not being perfect clamps and taking a finite amount of time to clamp. They also work with the input capacitance (either internal or added) as a filter to limit any voltage "spillover" from the clamping of the ESD diodes.