The reason Intel dragnets exist is that politicians always feel compelled to do SOMETHING after a domestic crisis, even if they know the ensuing compromise of civil liberties will accomplish nothing toward preventing the same crisis from recurring. But voters who don't pay attention (and don't really care) are appeased by bold reaction (and the thought that bad guys can't keep secrets from US). A symbolic knee-jerk usually violent reaction all they really wanted (like invading Iraq).
Frankly, after 75 years of 99% fruitless and increasingly expensive machinations by the OSS and CIA and NSA, all of us who are paying attention know know damn well that US Intel agencies really don't accomplish much that's useful to peacetime national security. That is, unless Intel's unblinking eye is turned toward targets who are of personal interest, domestically like MLK or PETA, or internationally like Saddam Hussein or Manuel Noriega. Then politicians unconcerned with due process like Nixon and Cheney can freely use that intel to attack their personal "enemies of the state".
30 years after the end of the cold war, THAT's why domestic Intel agencies continue to exist. All of the US military services have their own Intel agencies for use during war. They don't need the CIA or NSA. Only Presidents do. And IMHO, for all the wrong reasons.