Pardons by the executive branch are a weapon of peace. It exists to allow the chief executive, whether the President or a Governor of a State, to settle any matter in their jurisdiction.

At the Presidential level, the power is absolute because the founders believed in the executive branch being centralized in a single individual, the President. Outside of Senate confirmation, there isn't much that limits the powers of the President and it's like that on purpose. The people get to have their say every four years.