Well, realistically, it was just copied off powers that the British monarch had at the time. Lots of the oddities of the US presidential institution can be traced back to that.

Interestingly, this pattern repeats itself; countries which became independent from Britain later on often have a far less powerful president, with similar powers to when _they_ left. The president of Ireland, for instance, is non-executive, doesn't have a veto, and can't commute sentences... much like the British monarch when Ireland became independent.

As it stands, it's an anachronism that only survives because it's _really_ difficult to change the US constitution, I suspect.

That was only one of the reasons given in the Federalist Papers for its inclusion; the other is the need to be able to throw around pardons as a bargaining chip in quelling rebellions and civil disorders. The post-Civil-War pardons are a classic example - getting people off the hook for death-penalty treason stuff in order to keep them from going into a new rebellion in a few years - as are the post-Vietnam blanket pardons for draft dodgers.

It's a power Kings have. "The Royal Prerogative of Mercy". That's why the President has it, there weren't a lot of other examples for the Executive to be modelled on. However of course in modern constitutional monarchies the King doesn't operate this power any more, for example Liz's pardon power is operated by her government and ordinarily via some dusty committee looking into injustices (the government occasionally has used it directly e.g. some guy years into his life sentence for murder tackled a terrorist who was stabbing people at an event in London, the Pardon power was used to reduce his sentence in recognition of this bravery)

The US President should have handed this power over to a similarly dusty committee resolving real injustice years ago. This would be less corrupting and more effective because a President is busy whereas the committee would be doing nothing else except investigating the circumstances of potential injustice and choosing how to resolve them (new trial, pardon, etc.)