This library is intended to offer abstraction to the host OS’s underlying public interfaces. This is useful for OSes that provide interfaces in languages other than C/C++, or for grouping calls to OS-specific code so that it only needs to be updated in one place in the source tree. It is advisable to leverage existing functions provided by Qt, rather than rely upon OS-specific code, unless such code extends the application to provide a better OS-specific user experience or solve a problem.
As of Mac OS X 10.9 (Mavericks) many system public API calls to Carbon libraries (based upon C) have been deprecated in favor of modern Cocoa libraries (written in Objective-C), which can no longer be directly called from C++. Coalescing and mixing these new calls in a library, using Objective-C++ allows not only access to the Apple system Objective-C libraries and frameworks, but also those from third-parties, like the auto-updating Sparkle.framework.
See also: http://el-tramo.be/blog/mixing-cocoa-and-qt/ http://sparkle.andymatuschak.org/