Here's a hypothetical scenario: It's 2019, and the 45th President of the United States already has re-election on his or her mind. Somewhere inside NSA headquarters, there is an analyst who believes in his work, and sincerely believes that he has the nation's best interests at heart. This analyst thinks that some of 45POTUS's policies are leading the nation to disaster.
He can't wiretap the White House's phones, because that would just send up too many red flags. But he might be able to access cell phone records and browsing histories of the President's supporters, business associates, donors, that guy with dreadlocks whom the President knew in college, etc. And that analyst could potential find information that may not be incriminating, but it's still embarrassing.
You think a government employee won't try to blackmail the President? It not only can happen, it has happened before. Pick the worst President you can think of, and that's still not as bad as an unelected bureaucrat who doesn't have to worry about being fired or arrested.