Coroutines are when you don't want to run in parallel, because sequence matters. A typical example is iterators, you want the loop and iterator (routine and cor

Coroutines are when you don't want to run in parallel, because sequence matters. A typical example is iterators, you want the loop and iterator (routine and coroutine) to run in lockstep. You could use threads instead, but then you risk getting in all sorts of concurrency problems. And if you synchronize everything, then great, you have reinvented coroutines. In fact, you can think of coroutines as threads with built-in synchronization, which may be how they are implemented.