I'm creating a music player using C++ and QT, everyone says use FMOD. But QT has all the components, I can play audio, create playlist, and everything i need, so what does FMOD offer me, that can't be done with QT?
QT is a UI system, which has some capabilities to utilise media files, and if the inbuilt support suffices for your needs then by all means stick to just QT.
Fmod will however add a far greater audio capability for you and allows, as stated by Telastyn, a simple easy way to include support for diverse audio formats. This comes at a price and your application will be bigger and have more dependencies than if you only use QT, however its a case of horses for courses: which best suits your needs will determine which path you take.
Also, FMOD is a mature and well supported platform with new developments on a regular basis, and if done properly, your app can benefit from the updated features with little or no rewrite of code, just by replacing the FMOD dependency files you would gain any new format support FMOD has.