Most Qt Quick Apps need to register a C++ item into the QML context. According to the documentation, this is done with qmlRegisterType in main.cpp before loading the qml file.
However, the latest Qt Quick Application template is designed in a way so that the declarative root context isn't accessible from main.cpp anymore. Extending the qmlapplicationviewer.cpp/.h is necessary, which developers are not supposed to do.
More detailed description:
The fix proposed in the blog post is to add new methods to the template: