What good reason was it inserted in constitution that the president can pardon whoever he wants ?

On occasion there is a valid reason for the president to pardon someone but this has been totally abused by recent presidents. Just ask Bill Clinton. He pardoned hundreds of politically corrupt people who should be in jail. Sadly, political corruption is the norm these days at all levels and in both political parties. It's about who can rip off the American people the most and get away with it.