Allocation of distinct but equivalent controls within the three offices of the US Government is called Separation of Powers which is the unique aspect of the American constitution.
System of checks and balances allows scrutiny of one division over the other. It is appreciated by the whole world who desired complete democracy. This system ensures that no branch of the US Government becomes too influential while wielding their commands.
For example president can reject the laws that are passed by the congress and the congress can overrule the veto of the president if it is required. Thus the presidential power is delimited and all the three branches are given with equal value of responsibilities.