Hi! I wanted to give you a quick source to review while you wait to see if anyone else has points to add.

Federalist Paper #74 specifically discussed this power. Alexander Hamilton argued that having the ability to pardon allows the chief executive the ability to make exceptions to severe punishment and that it is better for it to be in the hands of one man, rather than a group. He also discussed how the issue of treason as a pardonable crime was debated in the Constitutional Convention and argued that having it as an ability might be helpful in reducing tensions.

I hope this gives you a reasonable starting place!

I agree with your first answerer, and while I can't specify how much it influenced the decision to give the president this power it's worth noting.

The 'royal prerogative of mercy' was a right of English monarchs officially since at least the 1600s. But even unofficially it was within the power of the monarch to grant reprieves. There were occasions where kings allowed their queens to plead for mercy of a condemned criminal. That way the king could show a stern face while also showing mercy. While the president wasn't intended to be a monarch, it isn't a surprise that members would seek to grant the president a right they were already well familiar with. Especially since the courts of the day didn't have the lengthy avenues of appeal that we have now, and when the country was smaller with less citizens, it had a greater impact to grant the president that power.