Reading the site and spec, it sounds like TextBundle is specifically for exporting and importing your Markdown (or other documents) to and from various apps or other users. In particular, it's needed for software in the Apple ecosystem because there are (apparently) strict rules on how applications can communicate with each other.

Personally, I was hoping TextBundle would be something like a standard format for inserting binary content into Markdown files. (Something better than just base64, anywho.) I could use such a thing for my home-grown wiki.