Good editors think on two wavelengths. They read like a journalist, like a news junkie who knows everything, catching errors and stylebook lapses. They also read like the general reader, who is smart but doesn't follow news the way we do. They look for papered-over cracks, omitted contexts, and simple failure to explain. Editing also makes the story read well, encouraging readers to learn more than a headline and a lead.
Reporters think it's because editors spend so much time in meetings, and they' re partially right.
Some newsrooms have been severely cut and don't have enough editors. In one newsroom, ignorant consultants classified every editor title as supervisory, declaring they had too many bosses for the number of workers, i.e., reporters. Their publisher fired the copy desk. Pagination and layout have absorbed so much of many editors' time that they don't have time to read stories. Some mid-level editors know their superiors critique only section fronts, so they make only those stories perfect.