tl;dr - the posts on the Briefly page are short snippets of text… not quite blog posts, not quite Tweets.
The appeal of owning your own content is not a new concept to me. I think I first saw the importance of it with bands like Metallica or Pearl Jam where having complete control over their entire catalogue meant that they could do whatever they wanted with it. Not only that, but because they owned the content, they were in a position to have a direct relationship with their fans without having to worry about an intermediary getting in the way. Similarly, many writers I follow speak about having your own domain name, running your own website, and avoiding things like Medium. The basic idea is that your content should be yours and you should be the only person making decisions about it.
I backed a project by Manton Reece called Micro.blog on Kickstarter because I thought a service like this might help me actually start blogging. It seemed like it would remove some of the barriers I have struggled with in writing on a website.
Unbeknownst to me, there is a whole movement focusing on owning your content that is referred to as the IndieWeb. It preaches owning your content, having control over the content, and being able to share it freely and Micro.blog seems to have very close ties with this ethos.
The Briefly page was created so that I would have a place to easily post short snippets of text that could then be cross-posted onto the Micro.blog website.