Today, the OS schedules a CPU for each thread, and might move threads between CPUs. But often, you have a heavy operation in one thread that you do not want to affect the operation of another (possibly GUI?) thread. In that case, it's very unfortunate if the OS decides to place both threads on the same CPU.

QThread could provide an API for determining the number of (logical?) CPUs, and for explicitly stating what CPU a thread should run on.