After a decade of using obj-c modules I'm still unclear on what the benefit of them is other than the Swift bridging aspects of it. There isn't really any observable difference in either compile-time performance or behavior from turning them on and off other than that a lot of the rules around inclusions are much stricter with modules enabled. The documentation claims that they should improve build performance, but AFAICT that isn't actually true or the difference is so small as to be irrelevant.
It's somewhat true and it should also improve debugger performance, but it takes a very long time to get to the point where that can be turned on and anyone sees it. (keyword is "explicit modules")
