That's a very interesting observation. I run without buds and the only time when I'd want them is really boring straight stretches, e.g. along highway.

I'd say novelty is important too. Whenever I do smth in new location or listen to new music, there's next to zero introspection. The usual location or known music seem helps introspection. In addition to that, if the location is unpleasant, it's hard to get into introspection mode.

TL;DR known-but-nice locations are best for introspection runs :)