If you find that there's some Apple software you want on your computer, but it's buried in a large package of stuff you don't want (or stuff that the installer won't let you install), Pacifist will bypass the barrier for you.
Pacifist certainly isn't new, but we've surprisingly never featured it here at Lifehacker, and if you're planning on trying out our latest and greatest Hackintosh build, Pacifist is definitely something you'll want. It will help you install software that Apple otherwise won't let you—whether because it doesn't recognize your computer as a real Mac, or because it's buried inside bigger, unnecessary installer packages. Thus, if an app that comes with OS X breaks for some reason, you can reinstall it without reinstalling the entire OS.
Another perfect example of a situation where Pacifist is necessary is the installation of iLife. Whether you're just doing a clean install of OS X or you're upgrading from a Mac to a Hackintosh, iLife won't be on your system when you first boot it up—even though you do technically own a version of iLife that came with your original Mac. The install discs that came with your last Mac won't install iLife for you, either—they'll only work on the Mac they originally came with. Pacifist, however, can open up any OS X install discs or installer package and let you install just the bits and pieces you want, and on any computer you want. It's really just a handy utility to have around, especially if you're running non-Mac hardware.
Pacifist is free as nagware, so it'll bug you to buy a license whenever you start it up—but you can keep using it for free as long as you put up with this.