Hi,
Imagine a high impedance source...a small solar panel for example.
With low light level it maybe can source 1mA with max5V.
Without capacitor it can only feed 1mA to the inductor, and only fir the time when the switch is ON.
When the switch is OFF, then the energy is lost.
During OFF time the energy stored in the inductance is transferred to the output side.
The stored energy is: W = 0.5 x I x I x L...but remember the current is only 1mA, therefore only a little bit if energy.
Now imagine with capacitor. All the current from the solar cell is fed into the capacitor. All the time.
The voltage of the capacitor rises.
Then the inductance is switched ON and a large current (maybe 100mA) can be drawn from the capacitor and fed into the inductance.
During this (short) time the current from the solar cell is still fed into the capacitor.
When the switch is OFF, then the energy is fed to the output...
Because the stored energy is much more than before the switching frequency is low...increasing the efficiency..
Klaus