Voltage Mode Control of a DC-DC Buck converter
Your schematic is showing current-mode control.  

In voltage-mode control the error amplifier compares the output voltage to a reference, then provides an output that is compensated and compared to a sawtooth wave that will vary the PWM duty cycle to drive the error lower.

At low frequencies (esp DC) we want the error to be as small as possible, so we want the error amp/compensation to look like an integrator.  Any error will then build up and drive the PWM to force the error to zero. Only zero error will allow the output of the compensation to reach steady state at DC.

If we just used an integrator for the compensation we would have to close the loop at a very low frequency which would mean poor transient response (poor rejection to disturbances).  That's because an integrator provides 90 degrees of phase shift and we would have to close the loop well below the L-C output filter resonance which provides another 180 degrees of phase shift.  We also wouldn't be able to control transients causing the output filter to ring because our control bandwidth would be lower than the LC resonant frequency.  Still, we want to put a pole at DC for good DC regulation.

The output filter L-C has 2 poles.  So in order to cancel those two poles we can put 2 zeroes in the compensation.  Now we still have that 90 degree phase shift until the switching frequency effects and error amp bandwidth start to come in.  

With the two zeroes we form in the compensator we get a couple of poles as well (if we want them or not).

So we put one additional pole in the compensation somewhere around half the switching frequency to filter out noise and ripple, and a pole at the capacitor ESR zero frequency to cancel that.  Now we can close the loop at a reasonable frequency with a reasonable phase margin and still get good DC accuracy.