The App Store is also full of these kinds of apps, promising you unrivaled productivity and writing bliss.
But, I would submit to you that all of these apps miss the point of markdown. From the markdown project page:
The overriding design goal for Markdown’s formatting syntax is to make it as readable as possible. The idea is that a Markdown-formatted document should be publishable as-is, as plain text, without looking like it’s been marked up with tags or formatting instructions.
The syntax (or lack thereof) of the markdown format is so simple that there is no need to “preview” the result. A heading looks like a heading. A list looks like a list.
This is a prime example of creating a solution to a problem that doesn’t exist. The reason why these apps are popular and people buy them has less to do with the need to preview markdown and more with the hisperized culture of writing for the web.