Author here.

> Maybe I'm missing the benefit here

Sundown is useful because it lets you abstract your custom server-side HTML tags. In other words, you can extend the language very easily.

For example, you can create a tag called "note" and use that instead of repeating HTLM, CSS and even JS:

    <s:tag name="note">
        <div style="color:red;">
            Note: {{@content}}!!
        </div>
    </s:tag>

And then call your new tag using the HTML syntax or Sundown:

    <note>Sundown is awesome ;)</note>
    :note Sundown is awesome :)

Result:

    <div style="color:red">Note: Sundown is awesome :)!!</div>

Sundown is just an HTML preprocessor (like Markdown) once it is transformed to HTML it's the SunSed language that compiles all the custom HTML tags to executable format.