I'll often have conversations with "nobody" while I work on the details of a post. Usually when driving, it's a good way for me to write & re-write based on how the ideas flow and inter-relate. It also results in some interesting cognitions, sometimes about totally unrelated topics.
I think it also helps with actual conversations with someone else to have already had a similar conversation with myself. Sometimes an impromptu conversation winds up with some long pauses while the idea percolates through my head.