I was having the same doubt till a few days ago. But after extensive research and looking at examples, I understood that Qt Quick is used in the apps where you need some good visual effects. So if you want some cool visual effects for an application, then Qt Quick is a good choice. Some places where one might need are embedded/mobile applications, few desktop applications like photo viewers, etc.

If you are developing an application for desktop/embedded platform which just need some basic menus and actions, then Qt-C++ is good enough. But this is my understanding about the topic.