Group brainstorming is most effective when at least some of the participants are prepared in advance, and maybe even written down something, and then sent to everyone attending.

Most won't read, but some will. Thus, we will have people with different levels of prior knowledge and preparedness. Variability helps in the Wisdom of the Crowd.

But I've also heard that some facilitators are best at brainstorming without any prior knowledge or preparation, i.e. starting with a clean slate. I think that way it's easier to ask naive or "stupid" questions that might lead to some interesting solutions.

In my experience, while group discussion establishes the context for a problem, breakthrough solutions usually come when thinking alone.