An important featue of the U.S. constitution is its provision for a system of checks and balances, the purpose of the checks and balances is so ensure that one branch of government does not become too powerfful, going hand in hand with separation of powers. Without checks and balances, all the power could go to one branch of government, ruling out the rest.
Checks and balances allow for presidential vetos. For example, in the judicial branch, the judicial system can rule an exectutive order unconstitutional, if it is deemed to be so, keeping the executive branch in line.