Anonymous sources get to smear smear people in service of their agenda without using their names. Without ever having to be scrutinized. Without ever having to answer a question. Without having to provide a shred of evidence. Meanwhile, the accused is left to prove a negative.
And then, the anonymous source story is reported by the rest of the media, and it's no longer "anonymous sources say" it becomes "The Atlantic is reporting." So most of the people who see it don't even know the sourcing.
There's no way that anyone thinks this is decent, worthwhile journalism. People only pretend to think it is because it vindicates their pre-existing beliefs about the subject of the story. The word for it is not "journalism." It's gossip.
For every Nick Sandmann or Anthony Scaramucci-Russia lie that's proven false, there are probably a hundred more that are never proven false because it's essentially impossible.