A big part of a forum is the centralization and interconnection of all the threads, this is explicitly removed in this product, with all the discussions happening in their own and very separate context.
So I guess the use case would be to share a link on social media and then have the discussion in something that is not as awful for real discussions as facebook threads or twitter conversations.
I'm not sure making a forum that simply has only one board and no public index is a useful enough differentiator for this project to survive at all. You could do the same thing with secret github gists, AND at the same time already have a much more fully-featured everything.
I'd see this being more useful to a non technical person who doesn't use Github for example and just wants to start a quick conversation. They can create a discussion and open it up to the world. Given a bit more time there will be more features to keep track of mentions and notifications that lift it up from it's currently very basic state.
That's a good way to look at it. The motivation is basically when you want to have a quick discussion about something but email or twitter aren't suitable. Like you mentioned twitter isn't great for real focussed conversations. It's also far more general than a forum, kind of like a forum to talk about literally anything.