It's part of the separation of powers and the system of checks & balances against powers of branches of government. Congress makes laws and impeaches president

It's part of the separation of powers and the system of checks & balances against powers of branches of government. Congress makes laws and impeaches presidents, courts judge constitutionality of laws and try cases of treason and presidents appoint judges and grant pardons. You can't have impeachment without pardon, otherwise, there wouldn't be a check against judicial tyranny.