Vary the integration factor of a opamp integrator
R1 and C, without R2, make an op-amp integrator. Adjust either value, of course R1 is easier to adjust than the C, to adjust the gain.

R2 does something else, it forms a time constant with C, turning it into a low pass filter, with low frequency gain R2/R1, and corner frequency controlled by the R2.C product.

Alternatively, you could regard it as an integrator that works for AC, and R2 limits the DC gain to a reasonable and controllable value, rather than infinity (theoretical) or the open loop gain of the op-amp (practical)