Hashtags are also used to make little tongue-in-cheek comments about the thing the user just tweeted, often mocking the tweeter and/or that tweet.
It seems reasonable to suppose (but I don’t believe I’ve ever heard anybody expressly assert this) that the use of hashtag to signal that a self-mocking (or otherwise ironic or “meta") comment is about to be made grew out of the categorization usage. I could easily imagine a user picking a label for a tag that would actually be useful for sorting the tweet, but that also happens to be amusing, ironic, self-mocking, or to otherwise provide a comment on the tweet. As in, if I tweeted about a silly WAG I came up with about a word’s origin, I could “tag” it with #justawag. This could actually be used to sort my tweets, but it would also serve as a wry acknowledgment of the uninformed nature of my speculative musing.
At some point (perhaps almost immediately, perhaps not) hashtags began being used in purely ironic/meta/jokey/self-mocking senses.
Things reached a truly meta state when Wil Wheaton of Star Trek: The Next Generation fame, attempted to post a link to some pictures, accidentally used the wrong link, and tweeted, “I love that I’m trying to be all clever, and then I epic fail at basic linking. #lessonsinhumility #facepalm #hashtag.”