STL is powerful but is often not sufficient. Qt is a great C++ API which propose plenty of tools (network, multithread, XML, ...).
The signal/slot mechanism is quite intersting. It allows you to do the same thing as function pointers and functors (except that you can't return anything from a signal) in an easy and pretty way.
The thread classes and methods are quite easy to use too.
The main reason that led me to compile it for the EV3 was to use a TCP/IP server class without reinventing the wheel. I wanted to command the EV3 remotely with an Android device. Of course, I could have used netcat to listen and redirect its output to my program but that seemed quite twisted.
I would say that if you want to code something and you know that it has probably been done before, then Qt is probably doing it already.
My blog: http://broija.blogspot.com/