[joconde]

If a program wants to perform a task in an async way without delegating it to an external program (like a database server or the OS' I/O system), it has to use threads, right? I think the point is that concurrency, for some tasks, basically requires multithreading. Not for the parallelism benefits, but just to be able to make concurrency possible for a task that requires blocking a thread.

[nightski]

Concurrency is more about having order independent units of computation. You can concurrently run operations on a single thread, although there is less benefit if no IO is involved. It's not something you'd likely do in practice unless there was IO.