I was thinking about if someone wanted to have the file pane closed, and had several bookmarks for whatever reason, they could do a search for query is:bookmarked and then find all bookmarks that talk about that topic.
But perhaps a better way for this would be to put results for items that are bookmarked on top of search results, the same way it is for documents?
I think it would be a rare use case, come to think of it, as hashtags would be better suited for the job and could be used for dozens of things.