The correct answer is that the Consul Veto of ancient Rome evolved into today's Presidential Veto, and that is how it contributed to the development of democratic principles.
The Roman Consul could veto the decisions of other Roman magistrates of lesser or equal hierarchy, so a Consul could veto what was done by a Praetor, but not on the contrary.
In the same way, the President is the one who holds the highest rank in a presidential democracy, being the Head of State. Therefore, it has the power to veto the laws emanated from the Congress that it considers negative for the population.
