Unfortunately, asynchronous programming is almost always discussed in terms of network I/O. However, there are many more use cases for concurrency. Coroutines can be extremely useful for modelling state machines or any kind of process that happens over time. Everytime you need to do X, then wait for N (milli)seconds, then do Y, etc., coroutines provide a very ergonomic solution. If your language supports stackful coroutines (e.g. Lua or Ruby), you don't even need to color your functions: you can just write regular functions that yield back to the scheduler anywhere in the call stack.

To give a concrete example: computer music languages, such as SuperCollider, need concurrency to implement musical scheduling. Imagine a musical sequence where you play a note, wait N beats, play another note, etc. Often you want to play many such sequences simultaneously. Stackful coroutines provide a very elegant solution to this problem. Every independent musical sequence can be modelled by a coroutine that yields everytime it needs to wait. The yielded value is interpreted by the scheduler as a delta time after which the function should be resumed. In this sense, SuperCollider users have been doing async programming since the early 2000s, long before it became mainstream.