Originally Posted by Thor 2
Pure speculation, but.....
I think the power of the king, and later the president, to pardon probably stems from a time where the most common sentence for any felony was hanging, and it was usually carried out within days or weeks of a conviction, and Courts of Appeals weren't really a thing.
I have to believe it was basically a way of overriding a verdict before someone could be killed. The prosecutor and judge held life and death power over the accused, and the only appeal was to the king.