It's a check and balance on a legal system that isn't infallible. Governors have the same power over state cases.
How indeed? How is it, for instance, that a government entity.... like say... the IRS.... has the power to target, specifically, right-leaning folks or groups? How is it that one political party has the power to procure a warrant to spy on their opposition during an election year and campaign? Oh wait....
so much for that "check and balance" shtick.