How do I feed in a mesh or something and it outputs an algorithmically generated slat furniture? This simple example would make things usable.
From there I think you could experiment with different widths, heights and depths and eventually come away able to build a slat-based piece of furniture with a text editor.
I don't think I could write something from the ground up in this language without a lot of work, but on the other hand I've seen people using CAD programs and the goofiness that happens when, for instance, they have a few hidden points they didn't know about. This format avoids all that pretty nicely.
VRML would have been a good choice: human readable, many CAD programs can import and export it, and there’s a web viewer available.
Almost the rest of the comment is just random observations that have little to do with your observation.
I think they did it this way because they are going for a bit of a Markdown approach. HEF looks somewhat more human readable than VRML once you learn the syntax and they have a basic tutorial on their home page.
I fed one of their example pieces into their online viewer and it loaded instantly. I was hoping for a parts list from it but I don't see one. Aha, that's because it's embedded in the .hec itself.
Not knowing VRML I'm thinking I could build something with .hec, and I can't build my way out of a paper bag.