This assembly contains all open-source dependencies that come with graphical user interface.
Besides that, it is very similar to bdep-oss and meant to be an optional add-on.
Motivation to doing this is to speed up continous integration, as installation on boxes that will never see a GUI is much faster when there are no big dependencies.
Especially QT also runs in non-gui mode with QtCore and a few other libraries, but providing those separately would be easy enough to do it when required.