Why is this markdown editor better than Medium's? It probably has a few more features but the design and UI looks like a clone of Medium.
What separates this is that you won't just get a dump of HTML from the editor. You'll get the content split into JSON https://github.com/bustle/mobiledoc-kit/blob/master/MOBILEDO...
This is beneficial if you plan to use your content in more than one place. For example, maybe the content is used on multiple websites, pulled into an e-mail newsletter, displayed on digital signage, integrated into an app or news services outside of your control. If you're interested in this idea, I suggest searching for the terms COPE or decoupled CMS.