I actually find brainstorming a great way to refine existing ideas I've come up with but for which I don't have a clear picture of the details. Granted, it's something I'll do by myself and come back to it over time until I feel like I finally have clarity of purpose and implementation, but it is still a case of sitting there just dumping out a huge bullet point list of ideas and refinements as I think about the idea. Not only that, but often it also helps me expand the idea in directions I hadn't thought of, and sometimes pivot the idea into something even better.