I've been tossing in hashtags on lemmy posts for some time. I actually started doing it on reddit also before that. They are so prevalent elsewhere, I just figured they may be useful for discoverability in these places someday too. And now that accounts here can be followed by Mastodon, for example, it's nice the my hashtags show up and are possibly useful elsewhere in the fediverse, even if not yet used here.
In my view hashtags allow for 'virtual communities' that potentially span platforms. The sub-reddit style of community is good for organization, management and moderation; but hashtags allow for crossing all boundaries with minimal friction; one doesn't have to find a group or "join" anything to be a part of a topic's conversation (or at least see what other people are saying, and where they are saying it). Obviously this is how they became popularly used on Twitter, and then appeared in other big closed systems. But those systems don't talk to each other, so your hashtags are trapped on each platform. They are even more potentially powerful in the fediverse as they can potentially span everywhere ActivityPub is federated. And in fact do span many fediverse platforms already.
It's just a shame hashtags look so ugly. It's a wonder they got traction on twitter at all, in the crude form that they did. It just goes to show how useful they proved to be.