This comment explains that main.qml should be added to files for a simple, statically-built Qt Quick application, otherwise the Qt Quick plugin won't be found. In addition, if there are QML files that contain imports that aren't in main.qml, presumably they'd all have to be added as well.

This wasn't necessary with qmake projects because it was all automated, probably via qmlimportscanner detecting the QML modules that the file imported.