[scott_s]

I wouldn't say calculations, as that implies, to me, a mathematical kernel. I can't think of any low-level calculations that are easier with concurrency. But some problems are most naturally solved with concurrency - that is, it's easier to architect the software with threads. Applications with GUIs are sometimes like this: it may be easier to have a main application thread that does the real computations, and a separate thread that handles all GUI work.

The pattern is that instead of one monolithic process that must know about and do everything, sometimes it's easier to think about several independent processes that only know about one domain, and make requests or give answers to other processes that know about other domains.