Anonymous Monk:

Yes, but that's done after all the work has been queued. So the main thread simply waits for all the work to complete before exiting.

In a "real" application, that would let your main thread collect some statistics and report before ending, to let you know what's going on.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

In this similar example, joining (reaping workers) is done after obtaining results.