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 basic purpose of having multiple threads is so you can start something new before finishing something old, that is, something that's already in process.
Let's take the example of a driver. A driver typically has two obligations: one is to talk to the hardware and the other is to talk to other processes. Generally, talking to the hardware is more time-critical than talking to other processes. When an interrupt comes in from the hardware, it needs to be serviced in a relatively small window of time—the driver shouldn't be busy at that moment talking to another process.
One way of fixing this problem is to choose a way of talking to other processes where this situation simply won't arise (e.g., don't send messages to another process such that you have to wait for acknowledgment, don't do any time-consuming processing on behalf of other processes, etc.).
Another way is to use two threads: a higher-priority thread that deals with the hardware and a lower-priority thread that talks to other processes. The lower-priority thread can be talking away to other processes without affecting the time-critical job at all, because when the interrupt occurs, the higher-priority thread will preempt the lower-priority thread and then handle the interrupt.
Although this approach does add the complication of controlling access to any common data structures between the two threads, QNX Neutrino provides synchronization tools such as mutexes (mutual exclusion locks), which can ensure exclusive access to any data shared between threads.