If the president could make treaties and appointments on his own without seeking approval from any other body, he/she could make their best buddy from college the Secretary of State, despite them possibly knowing nothing of the U.S. or foreign governments. Or, the president could make a treaty with a country that everyone in America dislikes, only because that country promised to sell the president land in exchange for a treaty. By going through the Senate, treaties and appointments are filtered to ensure that qualified and reasonable appointments and agreements are made.