Instances where you don't need to supply along with your content any rendering information. It's not the structure of the content or the presentation. Navigation is a separate concept. I actually put navigation and comprehension together. I think that's too general.
Navigation can be completely separate and imposed, like a site map.
Probably be more so with Web phones and Palm pilots and such. Some of these things are orthogonal, not parallel. The way you structure a document is how you navigate it, in some respects. One of the things I mean is that there are bits put into Web sites, that are navigational entities, that are unrelated semantically to the content. I think navigation is really important.