Clumsy writers do a lot of that, too. They unthinkingly follow the advice to say what you’re going to say, say it, and then say what you’ve said. The advice comes from classical rhetoric, and it makes sense for long orations: if a listener’s mind momentarily wanders, the passage she has missed is gone forever. It’s not as necessary in writing, where a reader can backtrack and look up what she’s missed. And it can be intrusive in classic style, which simulates a conversation.
