There has never been a good way to compile Classic Mac OS apps on modern OS X – for the most part, you were stuck using ancient tools, either Apple’s MPW or CodeWarrior, running in a VM of some sort. CodeWarrior, of course, is not free, and MPW only runs on Classic Mac OS, which is unstable at the best of times and downright nightmarish when trying to use it for development in an emulator like SheepShaver.
Enter ‘mpw’ (which I will refer to in lowercase throughout as something distinct from Apple’s MPW toolset).
With the same source file, and only a handful of #ifdefs, I could build the same app for 1984’s System 1.0 all the way up to the current release of OS X, Yosemite.
I really can’t think of a good reason to use this… except “because you can.” That said, it’s amazing that this works.