I assume learning to build interfaces in Javascript allows you to port that skill to both the desktop environment (via something like Electron) or web environment through a browser.
If only there would be a standard way of doing GUIs in HTML, but if you give the same GUI to be implemented to 10 Web developers you will get really different code, some would write it in HTML and CSS, other would maybe use angular, others would use react or similar, or if the project uses other backend language/frakework you will get different results depending on what PHP,Ruby,.NET,Java framework was used.
In Qt you use the Designer, you need to learn about the layout system if you want to do good UIs, but your GUI is saved in a file and then the c++/python code implements the event handlers and app logic.
Exactly this. Javascript frameworks allow overlap. I'm using the same tools and language to create a UI. Don't get me wrong I like QT apps and still tinker with them periodically. I will probably use them more once Go finds a way to make truly portable UI applications.
I guess that's cool if you mostly do web stuff. But there are tons of developers that use Python/C++ for embedded systems, OS, data science and other things that could make the same argument for Qt.
In Qt you use the Designer, you need to learn about the layout system if you want to do good UIs, but your GUI is saved in a file and then the c++/python code implements the event handlers and app logic.