How to approach this question?
The circuit is for a sinusoidal inverter. To do this we switch the full voltage on and off with varying pulse width to get a sinusoidal approximation.



Figure 1. PWM for a variable frequency drive. Image source: KEB America.

From Figure 1 we can see that there are two things going on.


There's a low frequency - 30 Hz in the top graph - alternation of the DC to the grid.
There's a high freqency pulse train with varying width pulses to generate the sine waveform on each half-cycle.


It should be clear that only one switch needs to be opened at high frequency during one half-cycle. The diagonally opposite one can remain closed.

The benefit is that switching losses are reduced. When a switch is fully open the current is zero so the power dissipation in the switch is P = VI = V &times; 0 = 0. When the switch is fully open the current may be high but the voltage is low - a volt or two - so the power is still low. The problem occurs during switching when the voltage and current are between the ideal states and power can shoot up momentarily. Having only two of the devices doing the high-frequency switching reduces the heating.