Ask HN: Aside from parallelism, what applications are there for concurency?

There's a clear difference between concurrency and parallelism but it seems to me that the chief source of interest in the one is as a means of achieving the other. It seems to me that most applications of concurrency that don't necessarily involve parallelism, such as simulations, would be simpler to write and maintain and arguably more performant using an old-style master loop rather than using threads or other concurrent programming technique. Am I correct, or is a master loop just a low-tech form of concurrency? Can anyone point to an application for concurrent programming that isn't ultimately ultimately attempting to achieve parallelism?