It's bizarre to me, an Australian, how the pardon power is used in the US. Our federal, state and territory executive governments all have a pardon power, inherited from English law, that is, formally, unlimited (like the US federally and indeed it's less restrictive than many US states for state crimes).
It is a power used very sparingly, even though legally it is unlimited - the state of New South Wales is, as far as I know, the only one which publishes details about uses of the pardon power; in an average year there are 0 successful pardon/commutation applicants, and it's an exceptionally merciful year if they grant 2 or more. Other states and the federal government may or may not be a bit more generous, but we're talking very small numbers. Most pardons are for reasons of unsafe convictions where for whatever reason no remaining avenues of appeal are available (rare, these days, because each state has introduced laws to enable post-conviction reviews).
Historically, particularly in the 19th century convict era, the pardon power was much more important, and was indeed abused for political reasons on a number of occasions, but it seems that for the most part it quietly exists in the background and only gets significant public attention once every blue moon for a high-profile murder case or similar.
What explains the difference? Is it the requirement for sign-off by the King's viceroys that prevents abuse? Collective Cabinet governance that is accountable to Parliament? Maybe our political culture means politicians' friends tend to end up in prison less often and thus there's less opportunity for the abuse of pardons specifically? It's not particularly clear to me - if anyone's got some good comparative studies send me links!