The BlackBerry PlayBook OS architecture consists of the QNX Neutrino microkernel and some number of cooperating processes. These processes communicate with each other via various forms of interprocess communication (IPC). Message passing is the primary form of IPC in Neutrino.
Now that we know more about priorities, we can talk about why you might want to use threads. We saw many good reasons for breaking things up into separate processes, but what's the purpose of a multithreaded process?
The modular architecture is apparent throughout the entire system: the Neutrino OS itself consists of a set of cooperating processes, as does an application. And each individual process can comprise several cooperating threads. What keeps everything together is the priority-based preemptive scheduling in Neutrino, which ensures that time-critical tasks are dealt with by the right thread or process at the right time.