There are good reasons, but those reasons only exist in some utopia once in a lifetime bullshit crap, like pardoning draft dodgers for Vietnam.

Apart from that it seems its always been used for absolute bullshit

The purpose is, ostensibly, to prevent some form of miscarriage of justice or an over harsh sentencing. Think the guy who they tried to charge with a felony for harmlessly throwing a sandwich at a federal officer, if they had managed to convict instead of failing to even get an indictment. Or it could also be used to retroactively forgive people convicted of breaking a law that has since been overturned. Like if they decriminalize weed possession, those already convicted while the law was in place don’t automatically get their sentences overturned.

But it is a power that should be rarely needed, judiciously applied, and have sensible guardrails on it. But the founders were confident that the people wouldnt elect self centered autocrats, that congress and the courts wouldn’t be filled with sycophants, opportunists, and cowards, and that the public wouldn’t stand for blatant corrupt uses of presidential powers. But here we are.

Frankly, if we can’t stop blatantly corrupt abuse of the pardon powers or even have basic limitations on it (like no self pardons) then that power should be amended out of the constitution.