The obvious benefit for me is that Qt provides cross platform GUI creation which is, after all, what it's designed for. Building your own frameworks is great if you have the man power to do it once, but I think there is a strong case for using something like Qt even if it locks you into using its data types.

If you're just writing a utility of some sort then sure, you probably don't need the kind of overhead that Qt dumps on you. That said, I've written very performant code using Qt's threading and concurrency libraries and it t was surprisingly easy. Think industrial image processing that needs to be done at 150Hz on two camera streams simultaneously.