For example, a great use case is to use WordPress as a backend for saving content to the database and Stratum for displaying that content without having to load the whole WordPress app. But as I said earlier, you can write "regular" WordPress themes as well.
As for the templating engine comparing to Twig, you cannot call procedural functions because it doesn't support PHP or expressions other than object and array member access because I designed the templating engine so that the logic is performed on PHP rather than on the views. The syntax of the views has been designed to resemble the original HTML/XML syntax. Twig filters in Stratum are Formatters. Twig/PHP includes are somehow similar to Components.
You can read the syntax here (unfinished): https://neblabs.com/documentation/views
Very often WordPress templates grow so much to the point comments are really needed.