Clemency is a failsafe for system failure. It’s a power held by governors and presidents to fix mistakes, to clean up when the courts get it wrong and convict and incarcerate the innocent, or over-sentence the guilty. Ideally, clemency is a tool where the poor and powerless can fight back against an overwhelming, all-powerful judicial system.
That’s the idealistic view. Clemency is more often used for political gain and/or for those lucky enough or wealthy enough to get access to the presidents and governors who hold the power.