Presidents have long claimed, however, that the constitutional principle of separation of powers implies that the Executive Branch has a privilege to resist certain encroachments by Congress and the judiciary, including some requests for information.
For example, in 1796, President Washington refused to comply with a request by the House of Representatives for documents relating to the negotiation of the then-recently adopted Jay Treaty with England.
The Senate alone plays a role in the ratification of treaties, Washington reasoned, and therefore the House had no legitimate claim to the material.
Accordingly, Washington provided the documents to the Senate but not the House.
Eleven years later, the issue of executive privilege arose in court. Counsel for Aaron Burr, on trial for treason, asked the court to issue a subpoena duces tecum–an order requiring the production of documents and other tangible items–against President Thomas Jefferson, who, it was thought, had in his possession a letter exonerating Burr.(Source)
The National Review
June 13, 2017
When Attorney General Jeff Sessions appears before the Senate Select Committee on Intelligence today, he may cite executive privilege to avoid answering certain questions.
You can expect congressional Democrats and their allies in the media to scream bloody murder about this, and contend this is an inappropriate claim to this power.
Why do we have executive privilege?
It’s been invoked going all the way back to George Washington; President Dwight Eisenhower summarized it pretty simply: “Any man who testifies as to the advice he gave me won’t be working for me that night.”
The theory behind this unique presidential power is simple: the president needs the best advice possible, and this means both the president and his advisers need to be able to speak to each other and discuss matters of state with confidentiality.
Sometimes the right course of action is not the popular one; those who speak to the president may not want their actual perspective revealed to the public.