Why should I learn Qt?

[stampede]

Qt is used for creating a GUI, is that all? Or there is something in Qt beyond creating a button, which makes it worth a try?

Qt has built-in cross-platform support for databases, multithreading, networking, xml, unit testing, opengl, scripting, multimedia, ...
I think the answer is simple

For what reasons you would prefer Qt over anything else?

Qt is the most comfortable and intuitive C++ framework I've ever used.