During ON time, you store energy in the inductor's magnetic field. During OFF time, it gets utilized to charge the output cap. In DCM you utilize all this energy.
Another point to consider is that in case of a synchronous buck converter, those have a MOSFET (/BJT/IGBT..) instead of the flyback diode. If you control this low side MOSFET to open for the entire OFF time, it can happen that the inductor current goes negative (turns direction) during OFF time, actually discharging the output cap. Luckily, in this case this energy will not be lost however. At the next ON time, the inductor will deplete this reverse magnetic field energy to charge the INPUT capacitor, by pushing the reverse current through the HIGH side MOSFET.
In case of an asynchronous converter, the flyback diode stops the process at zero current, and the inductor current can't reverse, because of the diode's orientation.
Was this what you were unsure about?
Good luck,
Gabor