last edited: Mon, 15 Apr 2019 16:58:22 +1200  
It's built like a post, but it doesn't end up in the feed/timeline/stream/network.

It could be like a private/semi-private/unlisted post off to the side of all the action, for, I dunno, discussion purposes? Ranting? Squee? Brainstorming? Bouncing fic ideas off of your bestie?

It's probably going to be something you want to refer back to easily - because it's not going to get lost in the feed.

Best of all, you can change the privacy settings, because its not something that gets flung to all corners of the fediverse.

Find more examples here.