The issue with democratic systems versus a republic or a constitutional monarchy, whereas there is a Magna Carta, or otherwise any guaranteed protection of the people's rights is that it can be voted away to a majority.
An unelected king that has total control over his subjects with no restraint of power or protection for the people will make none the difference but that in a democracy that person was elected to have total control over his subjects.
The disadvantage, or advantage, of a democracy, is that the majority will always rule over the minority. Regardless of there protected rights.
By threatening a veto, the President can persuade legislators to alter the content of the bill to be more acceptable to the President. Congress can override a veto by passing the act by a two-thirds vote in both the House and the Senate.
