Microblogs on /kbin are essentially short-form posts. There are times where microblogs revolve around a specific topic, but this is not always necessary. On /kbin, they are generally associated with keywords (hashtags) that allow a microblog to be sent to a magazine that targets/accepts the tag, which then appears in the microblog feed. However, on Mastodon/Twitter, you can simply write posts that get grouped with the posts of other users or your own previous posts without replying to a specific original post.
Writing microblogs and utilizing tagged keywords allows people to search for the information the writer deems fundamental to the topic they're writing about. This enhances discoverability and reduces noise, as you are able to quickly browse any and all posts that are specifically tagged with those keywords. Furthermore, microblogs may generally incorporate multiple identifying topics as you group together these keywords/tags.
For example, the @cats magazine will have an influx of posts on Saturdays from Mastodon instances which are tagged with #caturday. Writing microblogs in this way makes it so creating a thread every Saturday in @cats isn't necessary. This has a nice side effect of minimizing the need for megathreads that were found on Reddit.
Additionally, you can also search for (and click on) #japanesepractice to review exercises that I have completed in the past. There are additional posts in the @learnjapanese magazine that would not include this information. By using this tag, you can access the posts that specifically pertain to my studies.
This is in contrast to /kbin threads, where the content of the thread is the centralizing topic of discussion as opposed to the keywords.
For example: our replies are centered around what the OP has asked instead of any keywords in the original post or subsequent replies. However, /kbin is more unique than typical forums, as we can #tag the comments to achieve a similar functionality to microblogs. Honestly, I would like to see this feature be utilized more, but ç'est la vie.
ernest