This seems to be like the classic wxWidgets [1], i.e. it's an API that wraps the underlying platform's default toolkit. So on Windows it uses Windows' native controls, in Linux it seems to use GTK, and so on.
That means that the advantage is being able to write against one API, and get cross-platform compatibility, which can be nice. It also means (typically) being limited in what you can do to the least common denominator, or you (=the toolkit author) end up having to re-implement features from one platform that you want to expose but that are missing on some supported target(s). Or, of course, have an API with non-portable parts in it.
In any case, it means the "look and feel" is not the core feature of the API since that is going to be "like the target platform" and that is the point.
Given the origin, I guess Lua support is important too, here.