It just depends what you're doing / what you want.
If you're doing N different things that are all actively using the CPU, then yeah, you want real threads. But if you maybe have a bunch of different things "in flight" that are mostly just sitting around waiting for something to else to happen to make progress, then something like coroutines or green threads could make sense 'cuz of lower startup and memory overhead.
It can also be handy just for code structure. You want two things making forward progress at once, and instead of using callbacks and a bunch of variables to try to maintain state, you just use coroutines and the implicit state therein, but since you didn't go to real, executing-in-parallel threads, you don't have to worry about all the synchronization/race avoidance junk that comes from that (but of course, you also don't get the multi-core speedup).
If you're doing N different things that are all actively using the CPU, then yeah, you want real threads. But if you maybe have a bunch of different things "in flight" that are mostly just sitting around waiting for something to else to happen to make progress, then something like coroutines or green threads could make sense 'cuz of lower startup and memory overhead.
It can also be handy just for code structure. You want two things making forward progress at once, and instead of using callbacks and a bunch of variables to try to maintain state, you just use coroutines and the implicit state therein, but since you didn't go to real, executing-in-parallel threads, you don't have to worry about all the synchronization/race avoidance junk that comes from that (but of course, you also don't get the multi-core speedup).