Hey there!
In the constitution, a separation of powers is indicated in order to preserve democracy and make sure that the say of the people and the say of the elected representatives in the government remains the law.
This is reflected in the structure of the federal government due to the fact that we have a system of government that doesn't have one person in absolute power. Although the president is the one in charge, there's always checks and balances where acts can be vetoed by other branches of government based on voting and the say of the people.
Hope this helps!