That's the essence of co-routines vs. sub-routines. If you've ever used a Unix pipe you've used a limited form of coroutines. "ps | grep foo" tells process A, the shell, to fire up processes B (ps) and C (grep), and tells B to send results to C, not its caller A. B runs a bit and yields, returning some data via stdout. C runs a bit, reading the result via stdin, then yields to B and waits for B to return more data. Pipes are actually bidirectional, so its possible for C to send results to B when it yields, but off the top of my head I can't think of a real such example.

I suppose conceptually they're similar but you're oversimplifying how Unix pipes work to the extent that it's apples to oranges in comparison to async/coroutines.