It was a little difficult to wrap my head around at first, but for me its main benefit is this:
In many static site generators, if there is a page in a collection of pages (say, a product page) that you want to add a custom layout to (but without affecting other pages in that collection), you often need to create a separate template, or modify the default template to allow for this exception. With Soupault, page content is typically written in HTML and giving a specific page a custom layout is as simple as including an element in that page’s content file and it will be treated as a completely standalone page. In other words, a content file is not as different from a template as in typical static site generators.
This flexibility and HTML-first approach cuts down on complex or sprawling templates introduced to deal with many exceptions. And as a result, someone only needs to know HTML to make most edits to a website. Of course, you can make certain patterns more efficient with “widgets” (basically snippets to insert content dynamically) defined in the main configuration file, but it is not requisite.
At the end of the day, the website is mostly just HTML with a simple project directory structure, which makes it understandable to a wider audience, which is important to me since I want the website to be easily understandable for future maintainers of it.
Edit: Another benefit is that Soupault is a single statically-linked executable with no dependencies, and therefore can be downloaded via a simple link. This also extends the longevity of websites built with it (i.e. it is easy to get the development environment setup).