Documentation + examples are definitely lacking ATM. Will be rectified in short order. To answer your question now, the goal is pretty simple: enable python to be directly used in HTML templates.
For usage with another template engine, I see it as more of a convenience / speed factor. Quickly write frontend logic in python without reaching for a new (albeit similar) syntax.
empyt can also be used standalone, in situations where people want to avoid a templating engine altogether, and just want to use python on the frontend.