Coroutines are about decoupling OS-level threads from logical threads (where OS-level threads are already decoupled from physical threads). The reasoning is act

Coroutines are about decoupling OS-level threads from logical threads (where OS-level threads are already decoupled from physical threads). The reasoning is actually very similar to why OS-level threads do not correspond directly to physical threads. You may wonder why OS threads are not themselves sufficient, but it turns out there is a decent amount of overhead involved in managing huge numbers of OS threads. There are limits at which Linux does not perform well anymore, leading to problems like live locking. Also, if you can avoid context switching/cache invalidation by keeping a single OS-level thread busy, you get performance benefits even before you hit OS limits. So the userspace scheduling and green threading that coroutines enable allow you to increase concurrency without hitting these limits.