The Maw is supposed to be a prison for those who are irredeemable. In the real world we have prisons in order to lock dangerous people away, to punish them, and to find out if they can be rehabilitated and let back into society. This is one of the reasons why many people are against the death penalty. Because it robs people of the chance to redeem themselves.
But the souls in the Maw are deemed irredeemable. So why do they need the Maw, if the souls could just be destroyed?