bipolarity clamp ±(Vz + Vd)
It is also used across the capacitor in OPAMP integrator circuit to clamp the output to a know level and stop the OPAMP driving itself into saturation (and thus cause issue and delays if/when it is winding out of saturation)
This back-back connection can also be used to clip the sinusoidal signal depending upon i/p and Vz of diode.This can be extended further to generate square wave.