Executive Privilege dates back to George Washington
If you don't like it, too bad. The Constitution provides for 3 branches of government. In theory, they're supposed to balance one another through checks and balances. The concept of Executive Privilege, while not defined in the Constitution, is that a president should be able to solicit advice without having Congress step in every step of the way demanding to know what he was told in confidence. It serves to balance the power of the Legislative branch. Could you imagine the howls from Congress if the Executive branch demanded to know everything that happened there?
As the courts have ruled, there are limits to Executive Privilege. For example, a president can't hide behind it to cover criminal activity, as in the Nixon case.