It instead *facilitates* multiprocessing. If each of your apps is singly-threaded, then the OS can just arbitrate each of those threads to a processor as necessary. By breaking up the code into smaller chunks, the processor (singly or multiple) can more easily perform out-of-order execution and optimization routines to simplify the task at hand. Furthermore, it's an easier way to allocate resources. If you've got one processor that's 20% idle and another thread that would take 30% of its resources, then that processor wouldn't be able to easily handle the thread.

Basically, breaking up code into smaller fragments allows it to be more easily managed. Of course, there is a limit to how much this can be done before the ends no longer justify the means.

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