It is effectively a check on the judiciary branch in practice and a means by which the framers sought to allow the government to show mercy.
That said, this was a controversial inclusion into the Constitution at the time. There was a deep disagreement among the Founding Fathers on whether the power should be unlimited. At the Constitutional Convention of 1787, there was a proposal made by Roger Sherman to require Senate affirmation of a Presidential Pardon. It was defeated 8-1, but it became a hotbed issue as the document made its way to the States to be ratified individually.
The anti-Federalists were adamantly opposed to the unlimited Pardon power. Their arguments rested primarily on the idea that an unlimited pardon could and would lead back to tyranny. Edmund Randolph also believed that an unlimited pardon, specifically a power that could pardon treason, would lead to tyranny.
After it became clear that the pardon power was mostly supported and would be included in the final draft, without a Senate consent clause, the conversation shifted to whether there should be exceptions made for treason and/or impeachment.
However, “treason” was a funny word to the delegates. England has had a long history of using “treason” as an all-encompassing concept, a sort of umbrella law where if someone the government didn’t like hadn’t broken other laws they could still be brought up on charges of treason.
We know from Madison’s notes that there was much discussion about whether the President’s pardon power should include matters of treason. One suggestion was to reserve pardons for treason to the Senate, but this was shot down by Rufus King of MA and George Mason of VA who claimed respectively that the Senate was too politicized and already had too much power in the Constitution.
The debate among the state legislators was far more intense. Hamilton wrote in Federalist 74 that “one man appears to be a more eligible dispenser of the mercy of government, than a body of men”. This became the core argument of the Federalists in support of the pardon.
The arguments against the pardon were from the Anti-Federalists. Specifically, number 67, in which George Clinton argued the proposed Constitution provided the President with a power that would “tend either to the establishment of a vile and arbitrary aristocracy or monarchy”.
Clinton also wrote:
…His power of nomination and influence on all appointments; the strong posts in each state comprised within his superintendence, and garrisoned by troops under his direction; his control over the army, militia, and navy; the unrestrained power of granting pardons for treason which may be used to screen from punishment those whom he had secretly instigated to commit the crime, and thereby prevent a discovery of his own guilt; his duration in office for four years-these and various other principles evidently prove the truth of the position, that if the president is possessed of ambition, he has power and time sufficient to ruin his country.