My guess here is that it’s to ensure that __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ is in the stack trace for debugging purposes. It has __attribute__((no inline)) which backs up this idea.
If you notice, that function just goes and bounces to another function anyway, so it’s a form of trampoline which I can only think is there with such a verbose name to aid debugging. This would be especially helpful given that the function is calling a function pointer that has been registered from elsewhere and therefore that function may not have debugging symbols accessible.
Notice also the other similarly named functions which do similar things – it really looks like it’s there to aid in seeing what has happened from a backtrace. Keep in mind that this is core Mac OS X code and will show up in crash reports and process sample reports too.